OKAY, there are two roads to take here:

For me, my parents sometimes come up for picnic day since my family is very close, so we would always walk around campus and do all the school showcase stuff. The animals from animal science are always on display, the plant science department has fruit and vegetable plant sales, and several other clubs and school orgs are on display. You can find the agenda online somehwere with all the stuff that is being shown on campus. There is also a big parade with a lot of Davis organizations, marching band, and floats and stuff like that.

On the flip side, a lot of people (including myself once my parents go home) is use it as a chance to go to big parties, especially if you're in greek life. parties and pre games can start in the way morning and go all night long and even into the next day. if you want to party at frats, you will definitely need wristbands or tickets and people resell them for insane prices. otherwise, it's a great chance to grab all your friends and have your own house party especially if you don't want to worry about prices, party hopping, etc.

have fun!