When I did slimming world I always rewarded myself after going to Weigh-in with food which normally consisted of a takeaway & a tonne of chocolate. Since doing CD I've not wanted to do that at all and I've rewarded myself with shopping treats and beauty treatments (I need all the help I can get!). Today I bought myself a new coat in the Next Sale and the best part of all is that it is a size 14 and I haven't been that size since I probably was 14!
I don't think it matters how you treat yourself, it could be a quiet afternoon without the kids, a trip to the hairdressers, shopping etc, so long as it's not food, just so long as you know that you deserve it and you're worth treating. Whatever you do, enjoy!