I don't do anything formal in the summer months. My daughter will be in 4th grade next year and in the past couple of years we have allowed her to join a few weeklong camps of her choosing that are art, science, etc focused. She gets to explore and interest and also gets a little extra knowledge of those subjects in the summer.
Otherwise, we try to incorporate learning into our homelife. We do math as we cook- measurements are fractions, talk about freezing pt as we make icecream, etc. and do lots of wordplay. Anytime math comes up (grocery store- which is cheaper? the bigger box?) then we encourage her to help. When we do any traveling we talk about maps and distances and how the weather and landscape changes wherever we are going.
There are loads of opportunities in daily life to use as teaching opportunities. I generally follow her lead as well. If she gets really interested in why it is we have allergic reactions to bee stings for example, then we head to the internet and library, etc. and really dive into that. It's much easier to teach a child something they are interested and WILLING to learn, as I'm sure you know... use those as your starting points and branch out from there. They will provide you will themes without knowing it!
The libraries do a summer reading program that we always participate in as well. They offer coupons and prizes for things like swimming, skating, icecream etc as you read. We pick books she enjoys but also pull books from the suggested reading list for the summer. She makes lists all summer of the fun things she has done and journals about them, or writes/draws comics, and lists what she wants to do the next week, etc. We also have a mailbox set up to write notes back and forth. Long rides are good for simple writing and scrapbooking about a the road trip or vacation too.
Hope this gives you some ideas!!
:)