We have always had a tiny decorating budget, but, fortunately, we also have wildly creative people!
Find cheapo fabric at the Walmart in the dollar bin. Use this to define areas by stapling it to ceiling and "swooping" it. (An easy storytelling tent, or with a small platform built from scraps and painted, a stage area for a drama workshop). You can also staple the fabric to the wall to define an area. We had a jungly-looking print fabric we used as a backdrop for dramas.
"Oops" (returned) paint at the home improvement store is super cheap.
We painted a hallway in bright blue, then used "oops" paint of many colors to create fish and other sealife. We glued foam weather stripping into a circle shape and mounted it onto cardboard. This we then dipped into a seafoam colored paint, and printed bubbles on the wall. The whole thing is really cool, the kids love it, and a 100 foot hallway cost us maybe $30 to paint!
Remember to use space other than the walls for decorating. Hang things from the ceiling:windsocks, kites, stuffed animals, art projects, giant dice or markers (games), etc. Refrigerator boxes are a staple - we've used them for huts, ticket booths, Bible times houses, market stalls, quiet reading areas and many others.(Always make sure children can be seen and supervised).
Window paint is fun for the kids, can be changed at any time and will suit any theme. Swags over the windows are a super easy thing to do (even for non-sewers like me!). You can use fabric, fake ivy or other vines, long branches, ribbon, kites,fake flowers, anything you can put together to cover the tops of the windows for interest. Good luck - hope all turns out well! JOY