I hate when I do this – I save money somewhere and then think, oh, I didn’t buy that/go there so instead I can buy/do this cheaper thing.
In this case, the reward is the immediate saving! Sure, I deserve a reward for hitting my big goals but all the day-to-day actions (or non-actions) I take to get there should be habit.
If I’ve just managed to avoid an expense you know what I should do? Make myself a nice cup of tea, go for a walk, borrow a book from the library, read a blog.