My 18 month old absolutely loves her sandbox and water table. She spends at least an hour every day just shoveling sand into water and pouring water into sand (and all over the patio and herself). It's amazing to see all the ways she plays with just a couple of cups. I also like to give her a fat paintbrush and let her "paint" with water all over the backyard. Children at this age are so experimental and creative in their play, it's fascinating to watch. I bought her a water table that is off the ground so she can easily stand at it, but I really think she'd have just as much fun with a plain plastic tub or a big bucket.
She also has her own patch of flowers (nasturtiums - hearty, fast-growing, easy care and edible!) near her play area that she gets to water with her cup or watering can, and she spends quite a bit of time helping me in the veggie garden (helping usually means some sitting or stepping on the plants, but she's learning). She also loves to carry around and arrange rocks and sticks.
I'd like to get her something to climb or slide on this year, but not sure yet what's out there.