Here's a review and demonstration of a 5 gallon solar camp shower bag. Fill it wh water, let it warm in the sun, then take a shower. I used rainwater from my rainbarrel for this demonstration, but any water source could be used.
It worked excellently, though it did make the water smell like PVC and it was heavy when full, so finding a suitable place to hang it could be a challenge. I like this system and recommend it highly.