There are so many things to do in Lake Tahoe in May. Paddleboarding, hiking, kayaking, gondola rides, and much more. You can even float the Truckee River. There may even be skiing/snowboarding/sledding available in May!
If you are flying into SFO definitely stop by Ghirardelli Square. This is where we first said 'I love you' and where we got engaged. While you're here, be sure to explore more of the city (Fisherman's Wharf, Lombard Street, China Town, Union Square, etc.). San Francisco is about 1 hour and 30 minutes away from Napa, so plan for a day trip! On your way back, make sure you drive over the Golden Gate Bridge.