I just spend 3 nights in Colmar a few weeks ago and wow, it was so neat! I recommend going to the Tourist Information office and getting this walking tour map. I enjoyed following this and seeing the various sights. I also discovered on about stop #4 that there are QR codes on the information signs so you can follow the route on your phone if you have service.
https://reservation.tourisme-colmar.com/medias/documents/documentation/documentation-PLANCOLMAR-UK.pdf
One day I was pooped and took the tourist train and thought it was not worthwhile. It can't get to some of the sights in the pedestrian areas.
The Unterlinden Museum was excellent - wide range of Roman to current. No reservation needed when I was there in April although it was surprisingly busy. Do get the Audioguide as the Altarpiece needs some explanation.
Colmar was very busy when I was there the week after Easter. Lots of German tourists, few Americans but most everyone in the tourist sector speaks English. Lovely outdoor terraces which will be even more fun in June over April, lol!
I recommend testing yourselves intermittently thru your trip so you don't have any surprises if a negative test requirement still remains. I took 6 tests with me (non-proctored) and bought a 5-pack at a pharmacie in Paris. I had terrible allergies from all the tree pollen so kept testing as I had some symptoms which thankfully were just allergies.
Have a wonderful time!