Our favorite outdoor toy is hands down our sand and water table. Both kids love it! It's a bit messy (since the favorite thing to do is mush the sand and water together), but the hours of enjoyment they have are worth the mess.
We like to add some toys to the sand and water table too. We have trucks (dump trucks and digging trucks are great for the sand), buckets, boats, shovels, and seashells. The seashells are a big hit. They love to bury them and sift through the sand to find them as if they're on a treasure hunt, and Pinky likes to pretend they're mermaids swimming in the water (you have to love that imagination!)
But my favorite part of the sand and water table? It is the one toy that the kids never fight over. They share so well when they play with it. I don't know what it is - if it's just big enough for them to each have their own space or if it's just more fun when played together - but it is a great, quiet outdoor activity!