In my personal opinion, I love to receive ribbons or trophies, but it's true that they are not as useful as tack as prize.
Depending of the competition level I would choose one thing or another, but maybe the competitor prefer to receive tack because they will use it someday :)
Hope it helps!
a few of the things I have received are: brushes, hoof picks, buckets, shampoo, horse jewelry, liniment, homemade horse treats, saddle cleaner, and sponges. For some bigger prizes for champion and reserve you could give fly masks, basic halters, nice water bottles with the show name and champion or reserve, a lunch box with the show name and champion or reserve, and a wooden grooming box with a plaque hat has the show name and champion/reserve with cheap brushes in if.