Vouchers? Many people have many things and sometimes prizes, however good they seem to the buyer just duplicate what the winner has in his/her bag already. For example, I would think that many landscape photographers already have a set of grads and a big stopper (not that you'll be able to find a big stopper at the moment!) so, while appearing grateful and graceful in victory may well be wondering how to pass their prize on to get the best return.
I see from the link in your signature that the prizes in your wildlife competition are cash for the top 2 and novelty (presumably lens?) mugs for the 5 runners up. Are these mugs all the same manufacturer or will the winners get a choice to match their system? Odd as it may seem, I wouldn't use a Canon one and have no friends who use Canons to whom I could pass a mug on.
Yup, I reckon vouchers or even cash is the way to go, just as it is for children's presents. Maybe a small token prize (maybe a nice picture frame?) but the bulk in cold, hard folding!