This one is by far my favourite. Working off rewards is the reason why anybody in this world works. Take for example your parents. They work to get their salaries to ensure they can fulfil their and your needs. A personal favourite of mine was food. Not just eating but cooking was by far the best getaway. It doesn't have to be as complicated as getting an iPad but something as small as watching an episode of your favourite show can do the job. I did not quite use this before the last few months of the exam and took the reward before studying!