I need to have some sort of plan or we will go crazy!
-We usually do 2 paid activities -swim lessons (1/2 hour everyday for 2 weeks) and a camp (which means 3 hrs a day for 1 week). Those weeks I build my shedule around that.
-sign up for as many free library groups as we can - which may mean 3 or 4 hour long activities that I attend with the kids.
-For any days/weeks with no structure I put a loose plan in my head. We have a pool so that helps (though it can be a pain, too).
Example:
6am-9am - waking up, breakfast, getting dressed, household chores for me, inside play for the kids
9am-11am - play outside (no pool. bikes, sports stuff, swingset, etc. If lucky I can squeeze in some gardening/reading/phone time) OR do weekly errand (grocery store, etc)
11am-12pm - lunch
After lunch will be pool time or playdate or local 'free' activity
Local free activities we do are playgrounds, nature trails, beach, mall (if rainy), town petting zoo. I scan the local papers for ideas and often find free or low-cost workshop type stuff. Then I plan for some not-free activities like a trip to the aquarium, planetarium, fun-zone type place. We'll often do this with a friend.
Another thing I do is keep a mental list of activites we can when bored or that the boys can do together when I need to get something done. Examples may be water paints, a new sticker craft, plant seeds together, play with a toy they haven't in a long time (their Little People are stored in a bin in the attic and I pull them out every now and then and they play with them for hours), printables, bake cookies. Hmm, actually my list right now is plant marigold seeds, make icecream, and numbers traceables & other printable activites.
I am NOT saying that I do all this or this is how it goes everyday. But I need to have some ideas & structure or we will end up doing nothing and I will go crazy

)
Oh, another thing I do is a baby-sitting trade with a friend. We watch each others kids so we can have some free time for errands, etc. With both boys in preschool I never have to go grocery shopping with them both and I find when I have to it is maddening!!!!! We try to do it once a week but it's usually more like 2-3 times per month.