I'm glad some ideas have been helpful - your idea of the assault course sounds great!
My DS is only 2 (so is still at the eating crayons stage too), so I'm drawing on ideas from my girl guides!
I don't think water games have been mentioned yet. Have a running race with cups of water - who has the most in the bucket after 2 minutes?
Also, you can add cornflower to usual washing liquid bubble mixture to make sticky bubbles. Use string and a drinking straw to make a giant bubble wand and create huge bubbles (tip: lay wand in mixture, don't shake it).
Take air-dry clay out and make monster models using sticks, leaves, stones etc. pushed into the clay. Once they've made a few, if they like this sort of thing, they could make up a story/puppet show based on them.
Is there anywhere close for pond-dipping? All you need is a jam jar (perhaps a book on pond life from the library as well). Alternatively, bug hunting. Again, if they are into competitions, you can reward each found species with a sticker, and when they find a certain amount, they get a treat.
Really hope I don't sound patronising with these suggestions, just hope that they are helpful or can spark other ideas.