I'm sure you've all participated in Car Washes to raise money for charities/camps/etc, but for something different, try a Dog-Wash-A-Thon!
The idea is people bring their dogs along and have them washed for a fee or donation.
As you can imagine, this event is a lot of fun but also gets pretty wet and messy so choose your location wisely. You'll also need dog soap, dog brushes, baby baths, a few hoses, buckets, old towels, hair dryers, etc.
If you have younger people in your group, ensure they are adequately supervised. Ideally you should have someone who can train the group prior to the day on how to safely and effectively wash dogs too.
Also make sure you have someone taking photos (you may need a water proof camera!)