As a (somewhat) local, I may be able to offer some opinions...
The best LCS there was Pioneer Cards. It's very close to the Museum and one of the few that has good prices. That's the one that is mostly just cards. The one directly next door has excellent memorabilia.
For lunch, I recommend Mickey's Place. Great chili. Met Yogi Berra outside at the Kirby Puckett/Dave Winfield/Etc inductions.
Time of year: Induction season, if you really want to get the electric atmosphere of the town and baseball lovers in every direction. To save money, we camped at a KOA in the area.
Other times would be the dog days of summer and early fall. Good baseball atmosphere then, but without the induction-size crowds.
Or Spring, before the shops have been cleaned out.
There is a folk/county/art museum in the area...your girlfriend may be into that (m ine was an intern there). It's called the Fenimore Museum.
The closest mall is in Oneonta, which isn't worth the drive. At all. I lived there for four years as a geology major at the State College.