The thief left few clues behind, so we have almost nothing to go on. We found a single muddy footprint beside the cash register. Besides the missing chocolate chip cookies, a giant wedding cake was also stolen!

Few means "not many" (emphasizing a lack of something), which matches the context of having "almost nothing to go on." "A few" means "some," and "little" is only used with uncountable nouns.

Beside is a preposition of place meaning "next to" the cash register.

Besides means "in addition to" or "apart from" the missing cookies.