I would do Paris, Belgium, and Amsterdam (any order) for a few reasons.
1) London, while amazing, is super expensive. So is Paris. So pick one of the two, and clearly it's Paris. You can easily spend a week or even a lifetime in Paris. (and I say this as someone who adores London and who doesn't feel the need to return to Paris).
2) Flights. Delta and its partners (KLM, AirFrance) have something like 5 direct flights per day to Amsterdam and at least 2 or 3 to Paris from Atlanta. That gives you a lot of flexibility in terms of air travel on both ends. Fly into one, end in the other. I would recommend looking online, then contacting Delta, KLM, and Air France directly. Be sure to use a private browser window.
3) Aesthetics = Belgium. Bruges gives you the smaller, old city, and it isn't super expensive. Ghent is also great. We did a Brussels-Ghent-Bruges trip for 8 days and felt like we got a good sense of each. They are easy access from one to the next--just a very short train trip. All very different, all interesting and pretty. Do a beer and chocolate tour for sure!
4) Amsterdam itself is great. But try off-the-beaten-track a bit with places like Haarlem or a Frisian town like Hindeloopen for a likely cheaper and definitely less crowded experience. Haarlem is bigger, easier to reach and get too and from Amsterdam, and is bigger. Hindeloopen is a quaint Frisian village, and they are quick to remind you there that they are not Hollanders--they are Dutch, but they also have a somewhat different language and culture. There are likely other similar villages you could visit, maybe even one of the East Frisian islands--we just went to Hindeloopen with friends, so I know it.
5) Food--obviously Paris, but Amsterdam is surprisingly good. They colonised Indonesia, so their food from that region is really good (or it was last time I went, which was, admittedly, awhile ago). And the aforementioned beer and chocolate in Belgium. Certainly London wins the food scene (imo even over Paris), but it is so expensive. Also, even after a week you would barely scratch the surface of London and feel like you needed to go back.
I am a London junkie. It's one of the few cities where I could see myself living if I ever left Hamburg, and I never feel like I have enough time there. And still I would think for this trip, you should consider skipping it for the reasons stated above.
Just to add--you will have a great time wherever you end up going, and it is sure to be perfect because you are doing it together.
That's my thoughts. I can try to find the hotel where we stayed in Bruges. It was good enough to recommend, though I don't remember the name. If I find it, I can post it.