I only ever heard of Spectaculum but I've never been there and I don't even know if they still exist.
Spectaculum, Frankfurt am Main
STOLTZESTR. 11
60311 Frankfurt am Mai
Spectaculum's long gone. There still should be T3 - Terminal Entertainment on Grosse Eschenheimer Strasse 41a, near Hauptwache subway station and just off the Zeil shopping area. That used to be a nice combination of comic book (ground floor) and game store (basement), but I admit it's been some years since I last visited. It's still around, though.
Apart from that not too much left. There's a Games Workshop store near Konstablerwache station, but that was always rather run-of-the-mill.
As for museums, there's a
narrow-gauge railway museum , but I've never been there myself. The city museum is so-so, the art museums are good, if a bit, well, art-focused; there's also a rather nice museum of mail services and an excellent natural history museum, the Senckenberg.
Outside of Frankfurt, if you have a car and don't mind ca. 30-45 minutes of a drive, there's the
Saalburg, a reconstructed Roman auxiliary cohort fort. The reconstruction mainly dates back to the early 20th century and is a bit iffy in places, but the exhibition is interesting and scientifically up-to-date. A visit in autumn can be quite interesting for a taste of the Roman frontier.