I have them too - just not done anything with them yet!
I have always fancied something like the Poundland £10 challenge... how much scenery could you build with a budget of £10 and a trip to a pound store?
Rules would need to be agreed - glues and tools not in budget, but what about bases or small decorativr bits, but that sort of thing. Never quite got round to feeling inspired enough to organise anything!