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.
The best play space of all may be an empty expanse. Give your kids a roll or two of painter's tape and let their sense of make- believe take over. Roads and farms, four-square courts and hopscotch boards: anything is possible.