Many people say if you need to make excuses they are not really your friends but for me, it's more that they just don't understand.
By allowing myself special occasions and events, I don't feel like I'm depriving myself. I still go out and have a good time but limit myself and make goals before hand. Also, this way I can look forward to the next time. For me, this is easier to do with drinking but even sober I have trouble with late night eating with friends on weekends. It's like I get second hand munchies!
Go out dancing there is often a dance area at college parties well get on it burns a buch of calories and otherwise you wiould be standing around drinking . Drink diet coke (anything in a red solo cup looks like you are drinking so water in a red solo cup is fine too). Offer to be the DD. Become a booze snob and only drink 1 or 2 of a really good booze instead and say it is because you prefer the good stuff.
Get involved with an intermural sport that has Saturday practice, or schedule a standing tennis / volleyball / ultimate frisbee game with your friends. For me, it used to be about just getting awaaaaay from the food. Even a shopping trip or a trip to the dog park (I didn't have one in school, so I looooved getting to see them... I missed my family dog so badly!) will get you out of the apartment, away from the food, and focused on something else.
If you don't want to spend a lot of money, make a pinterest album full of crafty projects that you want to try, and do one each weekend! They have lots of cute ideas, and you can deck out your place! =)
(pinterest = nifty website... you can google for it, if you aren't familliar with it.)