What can someone do for the first time in San Francisco in one day?

There is a hop-on-hop-off bus. You can go over the bridge, and hit pretty much all of the sites. It's touristy, but hop on hop off is often the best way to see the most you can of a city in a limited time.

Go to any of the museums in Golden Gate Park. Eat in Chinatown.

It really depends on you. Are you interested in not seeing SF but being in nice museums instead? Would you rather be outside and get a feel for SF? Are you planning on walking a lot? Do you you want views? Do you just want to be near the water?

The early morning on the waterfront, the rest of the day in Chinatown, which is a trolley ride up the hill from the waterfront.

Tramway.

San Francisco, like Budapest or Istanbul is famous for its thermal Bath Houses, and San Franciscans often relax after work meeting up for a drink and party in these. Most are found in the South of Market area.