proksima

Two things to note. The prepositions differ by form: the adjective takes al, as in proksima al la rivero, while the adverb takes de, as in proksime de la fenestro. Both are correct in their place and swapping them sounds off. Second, proksima does duty for "next" in time as well as space, so la proksima semajno is next week and la proksima fojo is next time. Alproksimiĝi is to approach, common enough to have its own entry further down the list, and apud is the preposition when you want beside rather than merely near.