In the summertime, some church youth groups or non-profit organizations hold "donation" car washes. You could contact some church youth groups and see if they would like some for such an activity.
When we were kids, we played kick the can with anything that we could find--often using a 2-liter soda pop bottle with a little water for an off-sides kick. A 5 gallon bucket may be a little large, but as poor kids we would use just about anything for our "can".
Put lids on them, cut small holes and make bird houses out of them. They should hang nicely from a branch since they already have handles on them and will gently swing in the wind.
A Red Green style idea--I sometimes make 5 gallons of gatorade in the summertime to put in my cooler/dispenser if I have a big project planned (my cooler keeps it ice cold for 3 days). So you could add the mix in the 5 gallon bucket and water. Secure the lid onto it and them attach to your rim of your car/truck/or cub cadet using the handman's secret weapon--duct tape. Drive around until it is thoroughly mixed and then dump it into your cooler/dispenser or use it as your own cooler dispenser.
Great for holding surplus--salt for salt spreader or extra fertilizer.
Another Red Green style idea--Use duct tape to secure a pair of your boots to the bottoms of two buckets--one on each. Put them on and lace up the boots good. Then you have an instant pair of stilts for painting/dry walling a ceiling or even changing a light bulb.
Use them for additional seating for when the mother-in-law comes over for a visit or relatives come for Thanksgiving. I don't think they will overstay their welcome.
Yard pong!! Set them up in your yard on two opposite ends. Get a ball--small basketball, etc. and create your very own game of yard pong. Bottoms up and you can mind your P's and Q's!!
....I kinda like this idea myself....
Stack them up, about 36" or so, place them about 5' apart and place a board across them for some cub cadet limbo!! This would be more fun after the Yard pong.
....Wow, I have really went off on a tangent from my original idea of donating them to a youth group.....sorry.
Ok....that's all that I can think of for now.
Cub Cadet 123