I've run a couple as 'treasure hunt' type rides, which makes it fun for those involved. Give participants a map of the route and put 'clues' out along the way for them to spot - the team who finds the most 'clues' win. For my 'clues' I put different breeds of horse on A4 sheets, and pinned these to trees, noticeboards, phone boxes etc. (remember to go out and take them down afterwards).
Another one I have been involved in gave each team an egg to take with them as a suprise. It was suprising how many teams tried to carry the egg round the entire ride with them - with varying sucess! I hid mine at the top of the yard drive, however broke it when mounting on the way back :-S
I organised one years ago, its not easy. First you have to find somewhere to start from with enough space for parking wagons, trailers etc. This also needs to be on a route suitable for a ride, little or no road work.
You then need to rope people in to man checkpoints. I provided drinks for horses and riders at each.
Get insurance cover.
We then had someone to check the horses and riders were fit to start out. For example were the horses sound with no loose shoes, riders suitably dressed with saftety gear. We published these requirements in advance.
Have a trailer on hand with driver in case of accidents, you might need to pick a horse up on the route. The route needs to have some access points from a road.
Arrange with a vet to be on call if necessary.
Inform the landowners, even if you are using public rights of way.
Arrange for someone to provide food, a burger van or something.
Water and toilets needs to be available in the car park too.
Then of course you have to collect in sponsor money.
Its probably easier if you use one of the riding venues. They are insured and have all the facilities anyway.
It might be worth contacting EGB or one of the BHS riding clubs and run it in conjunction with them, as they will have all the relevant insurance and experience needed.