If you can not go on the river cruise, at least cross the river on the ferry. Ferries provide transport to the local people - they leave the jetty at Fancy bazar every half an hour and the round trip will take around 45 minutes. Tickets are Rs 10, 1 way. It is a simple way to enjoy the scenic beauty of the Brahmaputra river.
Compared to other cities in India, autos charge a hefty price for the ride. There is no meter system used by the autos, the auto driver will simply refuse to drive you if you insist on the meter - the best you can do is negotiate in advance check with a local on what it should cost you. Small distance charges are high.