Believe it or not, I'm considering Mannheim, partly because I it looks like I have to switch trains there on my way to Stuttgart.
Nice
The station is almost in the center of the city. You can get anywhere in the center by foot within one our (relaxed walking). The Zepyr Museum Block C 4.9 is about 15-20 minutes walk from the station.
A nice short tour would be to head for the "Wasserturm" (water tower) then into the "Planken" the core of the city and shopping mile. There are lots of little cafes and ice cream Parlours (tipp: "Eis Fontanella", one of the best there). On the way don't forget to get a "Brezel" (freshly baken Pretzels) from one of the little movable houses which sell them (a iconic Mannheim thing). Then you reach the Paradeplatz, tram central. Then you might head for the Zephyr Museum and after that back to the Station. Parallel to the "Planken" is the "Fressgasse" (munch-lane) you'll find all kinds of Halal food to-go there. Most of it quite good (even better food can be had in the area around H1/G2/I1 "little Istanbul").
Don't bother visiting the "Benz" house where Benz invented the Otto-Motor and the first automobile. It's very unspectacular. Basically a metal plate on the outside wall of a house.
If you have more time (like a whole day) the Luisenpark and telecommunication tower is worth a visit. In the park you actually can pet fish. If weather is bad: Technik-Museum and Planetarium (not good walking distance from central station, so taking the tram is the ticket here). If you have a car or take a taxi, the industrial complexes around the harbour area offer some photo opportunities. You will probably then also enjoy the typical Mannheim smell: exhaust from the "Schokinag" a nearby Chocolate-Factory. Its a very distinct smell.
Worms is quite unspectacular. Typical example of a city which was almost completely destroyed in WWII and rebuilt with ugly architecture. The tourist destinations are the "Dom" (big cathedral) and the Stadtmauer (city wall) or what's left of it and the "Luther Denkmal" (Luther memorial). Ice cream parlor tip: Vannini (at the market place vis a vis "Haus zur Münze"). If time permits, theres a small strip at the river Rhine just unter the bridge with some restaurants. They have a Biergarten "Hagenbräu" where you get a good "halbes Hähnchen mit pommes" (half of a chicken with fries) and a sandbar serving cocktails (not sure if it's already open though). Ah, an the Hagen-Denkmal is also there (a statue of the mystical figure Hagen trowing the treasure in the river...).
Have a nice stay
~andi