Sounds like you have an excellent list going.
I myself just picked up a job as a nanny for an 11 month old on top of teaching at an art summer camp for elementary school kids. So I too have started a massive list of things to do.
Now if you don't have access to a car to take the kids places, that's probably a good thing, with that many kids it'd probably be a handful. But if you do that's great, parks are the best place to take kids that doesn't require money and lets them run around and wear themselves out.
Here are some of the things that aren't too messy, fun to do, and are easy clean up.
Hemp jewelry making - this one is a massive hit in every class I teach
Dioramas of scenery are great if you have a lot of scrap material like fabric, tissue paper, wrapping paper and pipe cleaners and whatnot.
Nature scrapbooks - use some ribbon to tie some construction paper and have the kids collect cleaves and sticks and other things they find outside or at the park to glue in there. Crayon rubbings are fun with this one too.
If you wanna play in the sprinkler, make it fun with some water games. A summer camp favorite is Red Light, Green Light only each kid gets a paper cup of water that they have to get to the other end with without spilling anything.
These are just a few of some time consuming things you can do. Remember that kids tend to get bored easily, but can get burned out fast if they have to much to do. Pick projects that will take up some time and have a mix between fun things that are exciting and new, and maybe just some down time relaxing projects.
You're going to want a break too so don't burn yourself out either and good luck!