This doesn't mean that you should go overboard and go for those takeaway or ready meals but you can enjoy a nice meal out. Just be sensible. For example have a really good quality grass fed beef burger, an authentic Italian pizza that is often very light on the cheese, or if you're vegetarian, Turkish food is often an excellent option with a variety of mezes to choose from.
I don't usually recommend or advise on alcohol but once again if you have been really good, there's nothing wrong with a nice glass of red wine with your meal. If you're not a fan of red then white is the next best option; however stay away from beer and spirits.
Enjoy your cheat meal!