Evening sunset cruises on the Brahmaputra leave from Fancy Bazaar ghat (and a couple of other piers) — large flat-bottomed boats with two decks, music, snacks and a 90-minute loop downriver past Umananda Island. Operators include Alfresco Grand and Mahabaahu Cruise. Tickets run ₹500–₹1,500 depending on whether you want dinner included.
It is touristy and you are not seeing wilderness — Guwahati's south bank flickers past with traffic and city lights — but the river itself is enormous and the breeze is welcome after a hot day. Better than any of the riverfront restaurants for an evening drink with a view. For something more ambitious, a few operators run multi-day Brahmaputra cruises east to Tezpur, Majuli and Dibrugarh.