Late August, September & October are the best times for getting in the ocean. The water is warm enough to go without a wetsuit. My sister and I often go body boarding at Mission Beach and we're there for hours in the water.
If you go during the summer, get there before or by 10:30 am or you're going to have to park some distance away and walk over. This beach is very popular and fills up fast. There are life guards on duty all week. Rip currents are a common occurrence but if you pay attention to the life guards it won't ruin your day. Don't be surprised when the occasional seal and/or pod of dolphins swim past you!
If you're interested in architecture, there are some interesting beach houses along the boardwalk. If you go really early around 5:30 or 6am for a morning walk or run, you may still see some people hanging out on a patio left over from an all night party.
Belmont Park is right on Mission Beach and is a little amusement park with a wooden roller coaster. There's a wave park where you can practice your surfing skills on mechanical waves. There are shops where you can get that beach towel or umbrella you forgot to pack as well as places to pick up those kitschy gifts for friends back home. Also, you'll find a good number of places to eat and drink, after four hours of playing in the water, you may need a margarita!