To find something more thrifty and mature I scoured ebay to find 'job lots' of badges. With my rewards budget fixed at £10, I found many ex-collectors offer deals on variety bags full of random badges. For example, the lot displayed to the right is of 50+ badges and is currently selling for £3.50. That's 7p a badge - almost as cheap as stickers!
At the beginning of this year I bought three lots for less than £10 and kept the badges in one of the boxes sent to me. When students deserve a reward they 'lucky dip' into the bag. Sometime the badge is brilliant; sometimes it's rubbish. Yet they appreciate the randomness and the individual nature of the prize.
Altneratively, if you like to give rewards regularly, keep with stickers but customise them with more grown-up phrasing. I have used http://www.thestickerfactory.co.uk/ every year I've been teaching. You can save your customised phrasings for future orders and their multi-image, multi-captioned stickers mean you can get LOTS of different phrases at a reasonable price.