I have a side job teaching french classes, where i make 60$ per week (cash). this is my "fun" money....i spend it on dinner out, coffee, bus fare, etc etc, and once it is gone for the week, no more spending. (which means walking where i need to go)
For the 5 hours i spend in the week teaching, it gives me piece of mind knowing i have that to be able to go out, buy a coffee, etc without worrying. the money from my grant goes to rent/groceries (modest groceries)/utilities. It makes me track my purchases and think ahead. Plus, if i am really busy and dont go out for a week or 2, I have a nice bit of money saved and can splurge on nicer beer or something. I really recommend doing this....it keeps my bank card in my wallet and makes me save.