Maybe a basket with various types of bubble blowers and a big container of bubbles?
Do you have room for a tiny kiddie pool they could splash in?
How about a big flower pot or some type of planter that you and they can plant some seeds in, water, and watch grow? Could be veggies, plants, flowers, etc. Kids love to plant and see things grow.
How about the grill play set, made by either Step 2 or Little Tikes, can't remember which, maybe both make one... then they could 'play grill' outdoors...
What surface is your patio? How about some sidewalk chalk? There are just endless possibilities what you can do with chalk... create a road that they can use cars and trucks on, draw pictures, learn letters and numbers, etc.
And we keep our smaller outdoor toys in a storage bin with a lid, so it keeps the stuff protected from the weather, and doesn't take up too much space.
I'm still trying to think of some of the other things my son has now or has had when he was a little smaller (he's 4). If I think of anything else, I'll add it... :)