Tap to pay. Any contactless card, Apple Pay or Google Pay works at BART and Muni gates. You don't need a Clipper card.
BART. SFO to 16th St Mission or downtown is about $11 and 25 to 30 minutes. Rides inside the city are about $2.50.
Muni. $2.85 a ride with tap or Clipper, $3 cash. The 1-day Visitor Passport is $15, which only pays off at 6 or more rides.
Uber or Lyft. Short hops of 1 to 2 miles run about $9 to $15, more in the 8am and 5 to 7pm surges. SFO to downtown is about $40 to $60. These are estimates, so check the app.
Bag storage. Bounce is about $5.90 a bag for the day, insured, near most BART stations. SFO’s own storage (International Terminal, Departures) is about $15 to $35 and open 06:00 to 23:00. If you are flying out late, store near a BART station on the SFO line instead and collect on the way back.
What I actually spent: $15 on bag storage, two train fares, and one ride to the last party. Walking the Market Street stretch kept the rest free.