Dangerous approaches

Monument Valley runway 34 is my favorite - F18 for the real challenge!

You can either stall above the ridge and drop in like a stone and stop the fall with afterburners at the right time or try and do a right hand turn at very high angle of attack. You may want to work up to that though with something like the C172 or B58.

Innsbruck is pretty nice (Orbx scenery) if you fly a traffic pattern with an airliner. You can easily do a pattern with the widebodys, too.

Default scenery (or plus free DLCs):

I can also recommend Arcata RWY 14 (sloped at the end) with something like the A320.

Monterey (short and sloped) with Q400 or A320, B737.

Bob Hope RWY 26 left base to final,

Palm Springs RNAV to RWY 13 R (constant descending turn),

Marble Canyon (thin runway with a funny taxiway leading onto the runway at an angle),

Aspen (Colorado, steep approach into valley, tricky go-around)

San Diego (terrain sloping down, scenic but easy)

Reno/Tahoe (terrain sloping down, scenic and also easy)

John Wayne Airport noise abatement departure RWY 20R (full power takeoff holding brakes, steep initial climb then quick level off, slight left turn, slow and quiet shallow climb, then once at shore line resume normal climb)

Kern Valley traffic patterns (surrounded by mountains)

Page Muni RWY 07/25 (really short - 490m or 1600ft)
Napa County RWY 36R (quite short)

Columbia Airport RWY 17 (terrain sloping down) and RWY 11 (grass, short)

Westover Field RWY 01 (somewhat on a plateau, terrain slopes up)

Placerville Airport RWY 05 (terrain slopes up, plateau)

South Lake Tahoe RWY 36 (mountains in the way for bigger aircraft)

Gansner Field RWY 07 (long final... mountains in the way, steep approach)
Boulder City Muni RWY 15 (long final, mountains in the way, steep approach)

Round Valley RWY 10 (mountain definitely in the way, you may have to use quick lift up if you use long final start position)

Agua Dulce RWY 22 (terrain sloping down)

Big Bear traffic patterns are also quite fun (in a valley)

In Switzerland:
Interlaken RWY 05 (tight turn at the end, quite hard with an airliner),

Alpnach runway 19 right hand traffic pattern (if the left hand one becomes too easy), may not be possible with an airliner but with the F18 for sure

Lausanne (sloped)

Grenchen (sloped a bit and somewhat short)

Saanen with an airliner or even with the 747, both directions are very challenging,

Ambri (there's no straight in really, the faster the plane the more difficult it gets)

Samedan (anything bigger than the C90 gets really challenging. Try a jet or an airliner there!)

Ecuvillens (somewhat sloped but also relatively short, nice height changes on approach to RWY 09)

Geneva RWY 05 right hand pattern with visibility at lowest setting, in the 737, at night.