+1 on the liability release form.
I ride in a huge charity ride here every year and they have everyone sign a release form.
Also a good idea to get the local LEO's involved as their cooperation can be more of a blessing than a burden. Especially once you decide on a route. They can help stop traffic if you need it at intersections. Plus it's just good to let them know there will be a group of bikes riding through their area and that you're a charity ride.
The ride I go on every year is for a place called the Little Angels Home here.
The Little Angels is a home for children and young adults with severe disabilities and complex medical needs. It is a state-of-the-art facility offering round-the-clock skilled nursing, therapeutic and habilitation services in a warm, caring environment.
The ride is huge every year. It is about an 80 mile run. There is a charge to ride but it all goes to the charity. They charge $15 if you register early or $30 the day of the ride.
If you wanted you can do donations instead of money. I used to go on the Toys for Tots every year and you had to bring a new toy and canned goods, as much as you wanted.
On the Little Angels run they have a party at the ending point and every one with a wristband, which you get when you pay at the start, gets a burger or hot dog, chips and drink. They also have a raffle, local vendors and a live band.
You don't have to get that extravagant though. You could easily do a poker run. That doesn't cost much and if you get a big crowd it can raise some good cash.
Anyway, I'm starting to ramble now. I just wanted to let you know what I've seen.
Hope it helped.