the hippo is in St K's dock till this weekend but presumably gone by the time you get there.
Walk along the river on the south end of Tower Bridge - right by the river on the west side or along Shad Thames on the east side, old warehouses now pricey flats and restaurants.
Or take a river cruise from Tower pier? I'm not sure if that three-four hours or three-quarters of an hour, if the former you could probably get a Thames Clipper down to Greenwich and back, or somewhere up river and back.
https://www.tfl.gov.uk/modes/river/