Brixton village is an amazing location for budget lunch/dinner (try to go during odd times, it's less busy). It's a place full of little food stalls and cafés and they are all yumm in your tumm - delicious! I can recommend the Thai place and burritos in the Mexican place.
Afterwards you can rock down to Electric Avenue (!), cross the street and see the amazing David Bowie mural.
Location: Tunstall Road, SW9 - 1 min walk from Brixton Station
Going out in Brixton is fun and cheap as well! On the night bus home we even got offered some crystal meth, but my friends declined the offer in a very polite way: No, but thank you very much. Would you like some chips?
Oh, I love how polite people are over here :)