For better or for worse, the most long lasting product of 1991 just may be Teenage Mutant Ninja Turtles 2: The Secret of the Ooze. This is due in part to the appearance of Vanilla Ice and his rendition of the Ninja Rap. The movie was rushed out less than a year after the original and was a big step down. It was the beginning of the end for the once hip shell-heads.
The other, and better, feature film of 1991 was the Rocketeer. Not a box office hit, the movie was actually a pretty decent depiction of life for a 1930's hero - which pretty much only required a jet pack and a mask. Imagine how someone with actual super powers or even just Batman's arsenal could dominate back then.
Lastly and leastly, 1991 didn't bring us a live action Power Pack television show. Although a pilot was made for the Marvel team, good luck ever seeing it.