Best one: a first aid/vet kit that could be attached to my saddle.
Others I've seen and thought were great: Stensgar Mountain gave all it's 50
milers SunShowers so their horses could have a nice warm shower after rides.
I thought it was a great idea!
I think things that are useful and one-size fits all are great, too (I won a
Patagonia fleece pullover last year as a BC award...Patagonia seems quite
interested in supporting endurance, as I have seen their stuff given at
several rides) . Also good: certificates for feed if you can get a dealer
to give you some, feed bags, VetWrap, shipping boots, halters, grooming
supplies, coolers, gel-filled "cool caps" for horse and/or rider, hats and
mugs with ride logo, etc are all things that ANYBODY can use. The last ride
i did gave out logoed gym bags for completions. Also useful.