Anything outside -- taking walks, riding bikes.
With the kids I have babysat, we've walked to get ice cream, we've walked to go feed ducks, we've walked to go see the sheep at a nearby farm.
Make up some kind of scavenger hunt outside or something that will take lots of time, or hide little toys around the yard.
for some crafts try:
www.creativekidsathome.com
www.kidprintables.com has a lot of coloring papers or writing paper you can print out.
At preschool we like to bring inside toys outside for something different (We need to always have an activity planned for outside) -- The kids love it when we bring Legos or the Animals or Magnifying glasses outside. Or we've done sidewalk chalk, floor puzzles, crayons & paper on the sidewalk.
Not sure what your house is like, but bring out water and have them "paint" with water on your house or garage, or on the driveway. If it's really hot outside turn it into a lesson about evaporating (if your kids are young).
Another thing we did -- we froze a whole bunch of small animals and toys in a bin of water, then gave the kids the block of ice with animals inside. We gave them little play tools and they hammered at the ice for a long time, trying to "rescue" the animals inside. You could do something like that outside, and the heat in the summer will help it go a little faster.
Make orange juice or other fruit juice popsicles/ice cubes.
That's it for now.