We had so much fun in the park with our Toy Car Wash. I filled a large under the bed storage box with water, a squirty bottle of washing up liquid, a couple of spray bottles and a bunch of sponges and cloths... plus some bvery grubby looking toy cars...
I made this 'sprinkler' from an old milk bottle, with holes punched in the lid - with the handle it was the perfect size for little hands to hold (and pour, and chase you with!)
I think this is one of my cheapest playtimes ever - I had to buy the water (needed a couple of big bottles for transporting to the park) but it was less than 80p for 8 litres, everything else was on hand and couple of enthusiastic toddlers thrown in to boot!
...even Transformers need a bath sometimes!
If you're not into toy cars, you could add some farm animals, baby dolls or even try building a Lego tower in the water!