As others have said, we follow standard tipping practices as the rest of the USA. Here's my general rule:
Taxi: 10% - 15%. Adjust up or down if your driver was good or bad
Airport shuttle: $1-$2. $1 if they just drive you. $2 or more if they help you load bags
Luggage hold: $2 for 1st bag and $1 for each additional
As far as things to do for a college student - depends what you like.
If you like museums, hit the SF Museum of Modern Art. If you like tourist stuff, hit Fisherman's Wharf and then take a cable car around town. If you like window shopping, go to Union Square. If you like to just hang out, go to the Haight-Ashbury district. If you enjoy some history, take the boat to Alcatraz Island...
Tell us what you like and we'll take good care of you