I sat _________________ Monica all evening, but she kept talking to the guy _________________ the candy stand..

"Beside" means "next to" and indicates physical position. "Besides" means "in addition to" or is a conjunctive adverb. The first blank describes where the speaker sat relative to Monica ("sat beside" = sat next to). The second blank describes the guy's position relative to the candy stand ("beside the candy stand" = next to it). Both require "beside."