Who: Prongs and Wormtail What: We haven't quite figured that part out yet. Pranking? Maybe. Warnings: Cursing, one imagines.
They were doing the shopping at the market near Peter's flat, because there was always the distinct, if slight, possibility of running into Sirius and Remus at the shop closer to James' place. Sirius probably took delight in shopping for himself. He took delight in nearly everything else.
"Oi, Prongs!" Peter called from down the aisle. "Which d'you think would be better for our purposes here? Chunky-style or creamy?"