Great suggestions so far.
Things along the lake may come with a bitter chill, and may not even be open. So the naval park, canalside, riverworks, etc. might not be at full roar like they would be in summer.
Letchworth is always awesome, no matter the time of year.
Niagara Falls as well. If you're confined to the US side, check out the New York Power Authority Power Vista. Sometimes they have cool stuff going on, and they aren't far from the falls (they always have an interesting demo on how the power of the river is turned into electric power).
Buffalo is also steeped in some serious history as one of the wealthiest cities in the US for a while. It was a hub of industry, and we are finally starting to showcase that.
Bottom line is, there's a ton of stuff to do....it all depends on what you like. If you're a teetotaler who only likes granola, I'm going to recommend something very different from a party animal who is terrified of being moire than 100 yards from a 100 story building. Similarly, there are some serious thrills for an outdoor buff, as well as a person who enjoys history tours and really high end art museums.
Buffalo has a lot to offer (surprisingly), so what would you like to get out of it?