1. Summer activity - Go to 17 miles/ Carmel for sightseeing or relaxation.
2. Go skiing in Lake Tahoe which is one of the most beautiful lakes in America.
Other major sigtseeing:
1. Grand Canyon National Park (take the private plane to go around the Canyon). The sight is really rewarding.
2. Crater Lake National Park in Oregon.
3. Yellowstone National Park in Utah.
4. Go to the Niagara Falls on the American side. It's a fantastic view of the famous waterfall.
5. Yosemite National Park in California. Also have a lot of waterfalls for sightseeing. The best time to go is in summer.
You can do hiking for all of the above, but Yosemite Park's hiking trail is quite tough.