Just bumping this thread again to thank you all again for the advice, we had an awesome trip! I'll add a couple other observations of my own now in case anyone searches for this later:
-In 2 weeks we did feel like we got to see a LOT of the country, but we could easily have been there longer and taken things a little slower and/or seen more... it's a small country but there is a lot to do and see, not to be under-estimated!
-We were very glad we chose to stay in Ghent rather than Bruges; although Bruges was very nice we felt pretty "done" there in one day trip. (It was also the most tourist-packed of all the places we went by far, which I suppose for some would be comforting since that does make you feel less awkward and in the way as a tourist. Just depends what experience you're looking for.)
-The countryside in the south is gorgeous, totally worth spending some time in for anyone who doesn't require the constant bustle of cities. We preferred the quiet, personally! Also, if you are in the area between Dinant/Namur, our AirBnB host recommended this and we thought it was really fun:
http://www.rail-bike.be/ (The guy who sold us our tickets sounded a little surprised to get a couple of Californians out there, and wondered how on earth we had found it, apparently they don't get a lot of Americans coming to do it.)
-A car was fantastic for the southern countryside half of the trip, but we were very glad not to have one for the city half (and super stressed out the day we drove into Luxembourg City). With the car, just getting from point A to B allowed us to pass through a lot of tiny little towns and parts of the country we might not have seen otherwise, and make occasional detours if we saw a signpost pointing to something that sounded cool.
-If you're into war history, the Menin Gate in Ypres and American Cemetery outside Luxembourg City are a couple of beautiful and moving places to pay respects to the fallen, and the newly spruced up museum at Bastogne is jam-packed with information and artifacts, almost to the point of being overwhelming when you first walk in.
-Man, I wish I had learned more French... :) So glad I at least learned what I did! We did fine, and most places we went there was at least one employee who could speak a little English if we needed to ask a more detailed question, but there were definitely a few times when it would have been great to be able to communicate better with some really friendly but French-only people. (And when jet-lagged, trying to decipher the train notifications on the way into Brussels from the airport which were only in French and Flemish.)
Overall, lots of fun, and very hard to come back to the real world and work! Helps solidify those financial goals getting a taste of freedom from the grind. :)