We understand you won't spend your entire stay basking in Drupal so here are some things in the area you may enjoy.
BTW, the weather in April is generally one of the nicest months of the year. Springtime in San Francisco is often warm, dry and beautiful. It would be advised to pack a rain coat or umbrella as you never know when its going to rain in San Francisco.
Local Drupalers are hard at work compiling the best concerts, special
events, music events, DJs and other activities afoot in San Francisco
during the week of DrupalCon. Look here for specific information on free and for-charge events in San Francisco and (slightly) beyond to make your trip to San Francisco for the Conference extra special. Events for kids are included in this section.
The Drupalgängers will be organizing a variety of adventures for spouses, partners and significant others during DrupalCon. Conterence registration is not needed. Just visit their DrupalCon SF thread for details.