Just ran through the list myself. A couple may be considered "cheating" because if I saw the same feature in a different location, I counted it, but I made notes on where I saw the feature. I'm at 48/100 sensu lato, 45/100 sensu stricto.
1. See an erupting volcano – Hawaii Volcanoes National Park, Pu`u `O`o Crater lava tubes.
2. See a glacier – Mt. Baker, Mt Rainier, etc.
3. See an active geyser such as those in Yellowstone, New Zealand or the type locality of Iceland – Yellowstone National Park
4. Visit the Cretaceous/Tertiary (KT) Boundary – Hell Creek Outcrops (Bug Creek and Makoshika state Park)
5. Observe (from a safe distance) a river whose discharge is above bankful stage – Small stream in the center of Bozeman. It was interesting when the wate reached the narrower culvert.
6. Explore a limestone cave - Lewis and Clark Caverns, Carlsbad Caverns, a couple in NY
7. Tour an open pit mine - Berkeley Pit & Luzenac Mine, MT
8. Explore a subsurface mine - I think it was the Sunshine Silver Mine, Coeur d'Alene Area, ID
9. See an ophiolite, such as the ophiolite complex in Oman or the Troodos complex on the Island Cyprus (if on a budget, try the Coast Ranges or Klamath Mountains of California). I have driven the coast of California, but I can’t say that I have seen a whole ophiolite complex.
10. An anorthosite complex, such as those in Labrador, the Adirondacks, and Niger (there's some anorthosite in southern California too).
11. A slot canyon - The Narrows in Zion National Park, Bishop Canyon near the San Rafael Swell UT, and Big Horn Canyon, UT.
13. An exfoliation dome, such as those in the Sierra Nevada.
14. A layered igneous intrusion, such as the Stillwater complex in Montana or the Skaergaard Complex in Eastern Greenland. – Only core from Stillwater, MT15. Coastlines along the leading and trailing edge of a tectonic plate
16. A ginkgo tree
17. Living and fossilized stromatolites – Belt Supergroup Rocks, Northwest MT18. A field of glacial erratics – Too many places to count
19. A caldera – Yellowstone Caldera & manyo f its past calderas
20. A sand dune more than 200 feet high – Bruneau Sand Dunes, ID
21. A fjord
22. A recently formed fault scarp – Does 1959 count? Cabin Creek Scarp at Quake Lake, MT.
23. A megabreccia – I’ve seen some really large clasts in breccias and conglomerates, but I don’t think they count as megabreccias in this case.
24. An actively accreting river delta
25. A natural bridge – Yellowstone National Park and Arches National Park
26. A large sinkhole Florida, lots of them there. Could also count collapsed lava tubes… I’ve seen lots of those in ID and HI.
27. A glacial outwash plain – I’m guessing this has to be an active glacial outwash plain?
28. A sea stack – I know I have seen them, but I can’t remember where, so I won’t count it.
29. A house-sized glacial erratic
30. An underground lake or river - A cave in NY and a disappearing river in FL.
31. The continental divide
32. Fluorescent and phosphorescent minerals – Butte Mineral Museum, MT
33. Petrified trees – Yellowstone again.
34. Lava tubes – Lots near Idaho Falls.
35. The Grand Canyon. All the way down. And back.
36. Meteor Crater, Arizona, also known as the Barringer Crater, to see an impact crater on a scale that is comprehensible – Saw it on the horizon.
37. The Great Barrier Reef, northeastern Australia, to see the largest coral reef in the world.
38. The Bay of Fundy, New Brunswick and Nova Scotia, Canada, to see the highest tides in the world (up to 16m) (this seems redundant with #21 fjord…)
39. The Waterpocket Fold, Utah, to see well exposed folds on a massive scale. I’m going to count this one because I have seen the San Rafael swell in UT.
40. The Banded Iron Formation, Michigan, to better appreciate the air you breathe.
41. The Snows of Kilimanjaro, Tanzania,
42. Lake Baikal, Siberia, to see the deepest lake in the world (1,620 m) with 20 percent of the Earth's fresh water.
43. Ayers Rock (known now by the Aboriginal name of Uluru), Australia. This inselberg of nearly vertical Precambrian strata is about 2.5 kilometers long and more than 350 meters high
44. Devil's Tower, northeastern Wyoming, to see a classic example of columnar jointing
45. The Alps.
46. Telescope Peak, in Death Valley National Park. From this spectacular summit you can look down onto the floor of Death Valley - 11,330 feet below.
47. The Li River, China, to see the fantastic tower karst that appears in much Chinese art. ---It’s on this years to-do list…
48. The Dalmation Coast of Croatia, to see the original Karst.
49. The Gorge of Bhagirathi, one of the sacred headwaters of the Ganges, in the Indian Himalayas, where the river flows from an ice tunnel beneath the Gangatori Glacier into a deep gorge.
50. The Goosenecks of the San Juan River, Utah, an impressive series of entrenched meanders.
51. Shiprock, New Mexico, to see a large volcanic neck. I have seen other volcanic necks with radiating dikes.
52. Land's End, Cornwall, Great Britain, for fractured granites that have feldspar crystals bigger than your fist.
53. Tierra del Fuego, Chile and Argentina, to see the Straights of Magellan and the southernmost tip of South America.
54. Mount St. Helens, Washington, to see the results of recent explosive volcanism.
55. The Giant's Causeway and the Antrim Plateau, Northern Ireland, to see polygonally fractured basaltic flows. Have seen several polygonally fractured basaltic flows near and in the Yellowstone area.
56. The Great Rift Valley in Africa.
57. The Matterhorn, along the Swiss/Italian border, to see the classic "horn".
58. The Carolina Bays, along the Carolinian and Georgian coastal plain
59. The Mima Mounds near Olympia, Washington
60. Siccar Point, Berwickshire, Scotland, where James Hutton (the "father" of modern geology) observed the classic unconformity
61. The moving rocks of Racetrack Playa in Death Valley
62. Yosemite Valley
63. Landscape Arch (or Delicate Arch) in Utah – I think there are much cooler things to see in Arches National Park… Just a personal opinion.
64. The Burgess Shale in British Columbia
65. The Channeled Scablands of central Washington
66. Bryce Canyon
67. Grand Prismatic Spring at Yellowstone
68. Monument Valley
69. The San Andreas fault
70. The dinosaur footprints in La Rioja, Spain
71. The volcanic landscapes of the Canary Islands
72. The Pyrennees Mountains
73. The Lime Caves at Karamea on the West Coast of New Zealand
74. Denali (an orogeny in progress)
75. A catastrophic mass wasting event
76. The giant crossbeds visible at Zion National Park
77. The black sand beaches in Hawaii (or the green sand-olivine beaches)
78. Barton Springs in Texas
79. Hells Canyon in Idaho
80. The Black Canyon of the Gunnison in Colorado
81. The Tunguska Impact site in Siberia
82. Feel an earthquake with a magnitude greater than 5.0 - So close, 4.6!
83. Find dinosaur footprints in situ - Seen not found…
84. Find a trilobite (or a dinosaur bone or any other fossil)
85. Find gold, however small the flake
86. Find a meteorite fragment
87. Experience a volcanic ashfall
88. Experience a sandstorm
89. See a tsunami
90. Witness a total solar eclipse
91. Witness a tornado firsthand
92. Witness a meteor storm, a term used to describe a particularly intense (1000+ per minute) meteor shower. Nope, just several meteor showers. Not up to storm level yet.
93. View Saturn and its moons through a respectable telescope.
94. See the Aurora borealis, otherwise known as the northern lights.
95. View a great naked-eye comet, an opportunity which occurs only a few times per century
96. See a lunar eclipse
97. View a distant galaxy through a large telescope
98. Experience a hurricane – In fact I have been through 5, the eye of 3 of them.
99. See noctilucent clouds
100. See the green flash – I’ve tried, but have not seen it.