DIY busy board full of switches, latches, and doo-dads for babies and toddlers to manipulate. This is great for fine motor skills. It is also a hazard for giving escapee toddlers the skills they need to get out unsupervised.
Take a 2 liter soda bottle, poke holes in it. Attach to a garden hose. Toss over a tree branch and let hang for a kids water sprinkler. You can loop it over a tree branch and let kids bat it around for more watery fun!