1. See an erupting volcano.
2. See a glacier. [More glaciers in the North Cascades than you can shake a stick at. The Winthrop Glacier on the north side of Mt. Rainier is IMHO the most dramatic with a huge, yawning mouth at its base that belches out a massive sub-glacial river. Falling rocks echo across the valley. Geology in action that is just jaw dropping.]
3. See an active geyser such as those in Yellowstone, New Zealand or the type locality of Iceland.
4. Visit the Cretaceous/Tertiary (KT) Boundary. Possible locations include Gubbio, Italy, Stevns Klint, Denmark, the Red Deer River Valley near Drumheller, Alberta.
5. Observe (from a safe distance) a river whose discharge is above bankful stage. [Nooksack River, WA]
6. Explore a limestone cave. Try Carlsbad Caverns in New Mexico, Lehman Caves in Great Basin National Park, or the caves of Kentucky or TAG (Tennessee, Alabama, and Georgia).
Also, I'm curious about what you think is missing. I'll add a few:
- A kimberlite pipe, not necessarily diamond-bearing. [I collected a sample of kimberlite from Kentucky. Not the friendliest place to go rock hounding. 'Nuff said.]
- Pegmatite insitu with "honkin' crystals"
- Turbidite sequences.
- Flint Ridge in Ohio, source of much of the Native American arrowheads east of the Mississippi.
- Soak in a natural hot spring.
- Sand boils from liquefied soils after an earthquake.
- Glacially striated rock outcrops.