Trier, Germany

We really enjoyed Trier. The Archeological Museum was fantastic, go see one of the Roman Baths, the Porta Nigra, the Cathedral, the throne room which is a church today and take a walking tour. It is also easy to visit Cochem and the Mosel from there. You could shorten it to 2 nights if you need to and pick and choose what to do, but you could also stay the 3 nights and 2 days.

Two nights is plenty. It is a nice place to relax, but to me one full day is sufficient to see the entire town.

Actually, I needed 2 full days/3 nights to see all the things jlkelman listed in post #1. I also enjoyed wandering around. I did miss a couple of things...the Roman Bridge, Karl Marx's house and a few other sights.

I met up with a Rick Steves tour here and some of the others on the tour had taken a short cruise on the Mosel River so if you've not done that on another German River that might be fun.

Burg Elz is nearby is you have a car. Not sure about public transport there.

https://www.trier-info.de/en

I guess it depends on whether you are having to cut days somewhere else as to how the 3 nights will work for you.

We met our Rick Steves GAS tour in Trier. Stayed there 3 nights- 1 on our own prior to the tour and 2 with the tour. There’s a great walkable central area with lots of restaurants and shopping. There should be enough to keep you busy for 2 full days. One of our tour mates even did a quick day trip to Luxembourg the day before our tour started.

We're headed to Amsterdam from Baden Baden in May and booked a hotel
for 3 nights in Trier.

How many nights in Baden-Baden? I see you're coming from Switzerland, right? Unless you intend to gamble or get yourselves a mud treatment there (Russ, erasing mental images from Rick's video where he does just that,) I'd stay only one night in B-B as a stopover (although maybe you'll still have time for one of those B-B activiities/inactivities anyway.)

If you're doing Trier... and if you're truly after relaxation... then shaving a night from Baden-Baden would give you 4 nights between B-B and A'dam. I like jlkelman's suggestion... "visit Cochem and the Mosel..." But I'd split that time, with 2 nights in Trier, then 2 in Cochem (or elsewhere) on the prettier part of the Mosel River. Cochem is on your route from Trier to Amsterdam anyway. Trier is great for a day or day and a half of sightseeing if you are into Roman history, but not terribly scenic; Cochem and Bernkastel OTOH are very attractive towns with wineries, terrific river scenery, and wonderful old-world buildings.

https://www.rlp-tourismus.com/en/holiday-regions/moselle-valley

Also have a look at these places:
Traben-Trarbach
Burg Eltz
Beilstein (catch a river cruise there from Cochem)

Map of the Mosel River between Trier and Koblenz

It's been a number of years, but we had one of our most unexpectedly enjoyable travel experiences there. While we were reading through Rick's self-guided tour in the main square, a local older gentleman approached us and asked us if we'd like a walking tour of some of the old Roman sites -- for free, he just liked to meet foreign visitors and practice his English. We had our own personal guide and history instructor, equipped with books with pictures rendering what some of the sites likely looked like in Roman times. He wouldn't take money, but he did let us buy him a beer at a delightful cafe and we sat outside talking about American and German politics and history. It was the kind of "meet the locals" serendipity that Rick encourages.

I think we only did 1 or 2 nights in Trier, which was enough, but I'm sure you'll find other things to see and do if you want to slow down and relax.

Read some of the other replies. I also recommend Cochem and the Mosel. Not sure when you're travelling but when we were there in early June (years ago), there was a great wine festival (if you go, be prepared for a lot of sing alongs to John Denver Country Roads though ;)