Or you could get off in Big Sur village, hang out there for a little bit, catch the 22 on the way back, and then stop at Pt. Lobos. Or Garrapata -- that's a good choice too. Pt. Lobos is pretty much all (very pretty) seashore, Garrapata has beach and hills access. One advantage to Pt. Lobos is if you don't want to stay the whole 4 hours, it's a 2 - 3 miles walk back to Carmel, or at least other bus routes and restaurants and stores. It's a mile or two more to downtown.
Nepenthe is basically a resort. Not much to do there except spend money, IMHO. It's a very nice place to spend money, though. The village has more to it (but not a lot), and less expensive options for food and stuff.
Yes, everyone says you should go to the aquarium
. It's a cool place, but if you're not interested in marine life and ecology, there might not be much interest. Some of the exhibits are pretty dramatic, though. It's at the far end of Cannery Row, which has been turned into Tourist Town USA. Other places to think about, if you're interested in historical sites, are Custom House plaza, Colton Hall and the cathedral, all in the general downtown Monterey area. Renting a kayak is fun, they give everyone a lesson before you go out. Adventures by the Sea is in Monterey on Del Monte Beach and at Lovers Point in Pacific Grove. It's fun to just hang out and watch the boats. You never know what's going to be there.
Do you have a particular interest in anything? Besides trains and buses, of course. Not much to see in that respect, although there's an old train line that goes along the coast, and through Cannery Row. One or two cars are on display, but never had much interest in them. The old Monterey station is a shop/deli now, not much to that.