Summer "Homework"

Since the boys were little we have done "homework" as our daily routine. I have usually just bought workbooks from anywhere I have seen them. The grocery store usually has a few as well as school supply stores. I have also bought quite a few from bookstores, especially Barnes and Noble, they have a big section. I really like teaching my children, (and I think that I am still living my childhood playing school :) I even have stamps that I stamp on their papers when they do a good job (you know as an incentive for them for them to do their best, and I am playing school, nerdy? Yep)

Anyway, after they start school, and gotten real homework, I haven't really done the workbooks during the school year, but I think it is a great idea for the summer. There is a TON of time to get everything done in the summer. Our days are usually unscheduled and open, so I just add them back in to their "stuff" that they have to do every day.
This is the first year that I have ever done "Summer Bridge" workbooks and so far I am really liking them. They have a set number of pages that you do every day for the whole summer, they have stickers in the back that you can put on the pages that the kids do to "pass them off". I have liked them. They have been a little more expensive than the ones I have gotten in the past, but I have thought they were great.
The boys really like doing them too. They don't ever even fight me on doing "homework". I think they actually really like it. It is also good that it is getting them ready for the grade that they are going into. I also have some plans to go to the library to learn about a subject each week. The boys have been telling me some things that they are interested in learning about and it is going to be fun! Frogs are first on our list.

Do any of you have things that you do or have heard of others doing in the summer to keep learning going? I would love it if people would share some of their ideas or great ideas that they have seen others do.