We went to Lands End park via the BART. It has some hiking and great views of the Golden Gate.
We took a day tour to Yosemite which is one of the most amazing places I've ever been, we were picked up at like 7am and didn't return till like 9pm, really fun day.
Muir woods was awesome as well and is like a 1/2 day trip, wasn't very busy when we went. Our tour stopped at the overlook at the other side of the GGB which was a great photo op. It also stopped in Sausalito for a few minutes.
Do not rent a car, we thought about it but after being there I would NEVER want to drive.
We stayed at the Hilton on O'Farrell using Priceline but didn't save a ton in May. The hotel was pretty good but not amazing, would stay there again though.
Thats my quick and dirty on our brief 3 day/ 4 night trip there in May. It was a great trip, have fun.