The bubbles always are a hit with little kids. And as you say, messy art projects are likely a no-go. What about a beanbag toss (sometimes called cornhole)? And setting up a pretty simple obstacle course that included small orange cones in a zig zag fashion,20" circles of rope, and a thin "balance beam" set basically on the ground can be fun and easy to put together.
Just make sure it doesn't last too long :) . And of course, water and snacks.