Will Haupt Bahn Hof have locker to store a suitcase for a few hours- Aachen

https://www.bahnhof.de/aachen-hbf/karte

Zoom in. The symbol of a suitcase with a key is the Schließfächer (lockers). They're just to the left as you enter the station building from the tracks.

There are 76 lockers at 9.4" x 16.6" x 29.9" (77 liters, almost twice as large as a std carryon, but only 9½ inches wide). 3,00 €.
Or 18 @ 44" x 61" x 93" (250 liters, huge). 4,00 €.

You will need coins for the locker(s).

I'm so glad you're going to visit Aachen. I loved it! The town is very charming and has a lovely vibe to it, with some very nice cafes and restaurants. It's smaller than Cologne and you could easily spend some time just walking around and enjoying the sites.

The cathedral is quite beautiful (especially the interior, which is drop-dead gorgeous) and houses the remains of Charlemagne, leader of the Holy Roman Empire. Aachen is one of the oldest cathedrals in Europe dating back to 800 CE. For over 500 years, it witnessed the coronation of 43 German kings and queens. It was also one of the first items to be listed on the UNESCO list of world heritage sites, mostly because of its artistry, architecture, and importance in the history of the Holy Roman Empire.

While you're in there, make sure you see the huge chandelier that hangs over the center of the cathedral chapel, the Pfalzkapelle, which is one of the oldest items in here. It was a gift to the cathedral from Emperor Friedrich Barbarossa around 1165.

Lastly, don't miss the Granus tower, which is next to the Rathaus (town hall) and is one of the few pieces remaining from Charlemagne’s palace. It dates back to the 8th century, making it over 1200 years old.

Aachen is indeed a neat place, and the Dom (cathedral) and Rathaus are both impressive. I would suggest an English guided tour for the Dom -- it lets you get up close to the gilded box that holds Charlemagne's remains and up close to "Charlemagne's throne," which probably was not Charlemagne's throne, but was the throne used for coronation of the Holy Roman Emperor over close to 600 years. Online tickets: https://www.aachenerdom.de/en/a-place-to-visit/#pll_switcher

The "free entry" line can be long at the Dom and is a quick walk-through. If interested in the tour, the tickets often sell out and may not be able to be purchased on day of arrival (as happened to the couple turned away right before my tour because tickets were sold out for the day).

The Rathaus is very impressive and there you can view the massive coronation hall where the post-coronation celebration was held. There is an entrance fee, but this history buff was thought it was worth it. There is a self-guided tour.