Leave plastic animals in a bin outside (near the garden, next to the sandbox, next to a patch of trees, etc) -- when children go outside to play, they can use them and let their imaginations run wild! My recommendation: the children should only play with these toys outside -- that way, they'll look forward to playing outside -- make sure they run around too ;)
let the children play: Be Reggio-Inspired: Outdoor Environments
"Kenan also cut a ton of natural wood blocks for the kids. They had a great time building fairy houses and structures and kingdoms and knocking them down again. Ours are really roughly cut (husband used a chain saw), we didn't sand them or seal them or anything. It is truly amazing to see their imaginations at work."