Down near Half Moon Bay is Ano Nuevo a beach where Elephant seals come to mate and give birth. It’s quite amazing to see them, although I think mating season might be over by April. There’s some amazing hikes around Muir woods, if you want to do an outdoor thing that’s not the beach. Mt Tamalpais is also fun, if the weather is nice. If you have a playful group of friends, spend part of the day in the Exploratorium—it’s not just for kids! Make sure you get to go into the Tactile Dome, it’s well worth it. Afterward you could do a beach afternoon/sunset thing, either at ocean beach or across the Golden Gate at the Marin Headlands. There’s some old military stuff around the Marin Headlands from when SF was a base for watching out for the Japanese in WWII. You mentioned white water rafting, which is always fun. I’ve only gone on the American river outside of Sacramento, I’ve never heard of trips on the Russian river (but that doesn’t mean they don’t exist).