Tesco have started a line in cheap trophies and medals that are helping get around this conundrum. In the 'party favours' section I picked up 4 trophies (like the one pictured) and 5 medals. Each pack was £1. Although they are made of plastic students audibly "ooooh-ed" when I pulled them from my bag and were enthused more than I have seen this year. When the prize stakes were upped to "a packet of hula-hoops and a trophy" their work-rate tripled!
Any other good ideas for 'grown-up' rewards for students? Please let me know at email@example.com