We take a break during the summers, for the most part. Things that continue: music lessons, any subject (if any) we didn't have time for during the year, and informal maintenance-type activities like math facts worksheets or computer games, mostly for the younger ones who tend to forget that stuff if we don't keep it up. The kids devour library books and participate in the summer reading program there. Other than that, they're out playing hard, swimming, hanging with friends, and catching fireflies.
This summer, we'll be doing science, since I don't feel like we did it justice during the year. Also, I'll be getting a head start on phonics/reading with my rising kindergartener...nothing intensive or high-pressured--just getting a head start so that when fall comes we'll be ahead of the game.
Spring (warm weather) comes early where we're from, and spring fever hits us hard. Summers are hot and humid...so, what I usually end up doing is ending the school year mid-April or so and starting back up in August in the dog days. Spring is just too delightful to miss, and August is, well, August.