Sapporo is not going to be pretty that time of year (early spring = lots of grey slush), and the city is not blessed with a huge amount of attractions. We were considering a trip ourselves in April, but a local friend emphatically recommended against it and told us to come in summer instead.
On the upside, it does have some pretty good eating, reasonably lively nightlife, the Sapporo Beer Museum, the Sapporo Beer Garden, and, uhh, that's about it, unless you're keen on checking out the local Eiffel replica and the exotic Western clocktower.