Advice - what to do for 2-3 days between Amsterdam & Cologne

Liege? Namur?

..Bruges in a heartbeat.

I'd spend the whole time in Amsterdam before moving down to Cologne. I don't tire of Holland, as it's so easy to travel there with the Dutch people speaking such good English, etc. And it's a very lively place for a 23 year old.

I agree with the previous poster about staying in Amsterdam until Saturday. Do you arrive there on Monday? If so, that day will probably be a wash due to jet lag anyway, so you'll have 4 full days there relatively jet lag-free to explore. Is there a reason why you don't want to spend all 4 days in Amsterdam?

Hamburg, Stuttgart, and the Black Forest are significant detours from the Amsterdam-Cologne train route. If you don't decide to stay in Amsterdam, then a smaller town in the Netherlands or Belgium sounds both more logical and more attractive.

I also recommend staying in Amsterdam at least a few more days then tag on the additional days to your Cologne visit. Do some research on each area. You will find much to do in both towns.
In Amsterdam, research Mike's Bikes bike tours. They offer both city tours and country tours. Great way to see the many unique areas if the city. My wife and I did the country tour and visited working farms, wind mills, parks and other sites. The tours are lead by young informed humorous guides. Your 23 yr old son would likely very much enjoy. Some students on our tour stopped and bought a 6 pack to enjoy during the breaks along the way.

Thanks for the recommendations, everyone! I think we'll do a few days in Amsterdam then a few in Bruge. Mikes Bike tours looks awesome. I'm thinking we may do the City tour on one day and the Country tour the next.