It is my home city, born and bred there 😊
There is lots to see and do OP, depending on your preferences.
You can go into The Dom, which is the cathedral, if you're feeling energetic you can climb the staircase to the top. Take a walk over the bridge where you will pass thousands of padlocks which declare the names of lovers, add your own if you wish, to the viewing tower where from the top on a clear day you can see the entire city.
You can take a boat trip down to Mosel, our wine producing region and stunning scenery.
There are many bars, cafes and restaurants, on colder days if you wish to sit outside those then blankets are provided.
Plenty of museums, history and ruins to keep you occupied if that's your thing, on a Saturday there is often a large antique / flea market.
Each major city in Germany has a nickname to represent it, Hamburg is the windiest due to its breezy location, Cologne is the friendliest, you get the idea. It is also Germany's gay capital.
The city is split into two towns. Old town and new town. Go over the bridge and there's not a great deal, but great for exploring, this side of the bridge is where the shops etc are, including a McDonalds in the middle of C&A.
Our local beer is Dom Kolsch, lovely. Enjoy it with a schnitzel. Many cafes and bars when you order a drink will mark your beermat, then you pay the bill at the end, just give the bar staff your mat and they will give you the bill. They will also keep you topped up, we hate to see empty glasses. Hah. Also enjoy a glass of model Riesling wine. There is an Irish bar, called Barney Valley if you fancy a Guinness instead.
There are lots of restaurants on the river front which is a short walk from the Dom, just go down the steps behind the Ludwig museum.
If you get on a train please get your ticket stamped prior to boarding. You will see Orange coloured boxes beside the ticket machines, insert ticket to stamp it, otherwise you will be travelling on an invalid ticket and will be fined by the inspector. Sounds silly as you have purchased the ticket, but it's how it is. Even if you purchase the ticket from the sale box, stamp it. It catches many tourists out. Even I got fined once when I forgot to stamp my ticket. Hah.
Unfortunately, you are too late now for Karneval, but please if you or anyone else decides to go to the event, don't book a package, it is much cheaper to just book a flight and accommodation. Some packages offer to pre book seats, which has me shaking my head in disbelief.
Oh, and have a ride on the beer bike. It's great fun.
Enjoy it. We love to see people having fun and enjoying themselves 😃
Hope this helps.