Watching the sunset over Lake Victoria is quite an experience. Down on the lakeside there are a lot of fish restaurants and guys trying to get you to go on a boat ride to see the hippos and crocodiles. It's also quite novel to watch everyone driving their cars and matatus into the lake to wash them clean and then drive through the dust back up to the road.
Our main reason for going was to look for soapstone carvings. There's a group of stalls on Jomo Kenyatta Highway near the Hotel Royale. Kisumu has the best variety and choice of soapstone crafts I saw during my stay, and the prices are a lot better than in Nairobi. Soapstone is quarried in nearby Kisii but not many travellers pass through there. There are also lots of good wood carvings, drums and jewellery.
Quick Tips:
Take lots of deet spray and a mosquito net in case your hotel doesn't have them. Kisumu has the worst rate of malaria in the country - mainly because of the lake I suppose. It's also quite humid. There are bird and impala sanctuaries to visit on the lakeside.
Best Way To Get Around:
Walking during the day is fine but unless you're in a group I'd take a taxi when it's dark. There are plenty of boda-bodas (bicycle taxis) if your feet get tired and taking one of these is quite a lot of fun although it's a bit dangerous.
There are loads of matatus and buses that go to Kisumu from other parts of Western Kenya (check the ones to Kisii as they'll probably stop on the way). Or share taxis are a slightly more expensive but quicker option.