I was there the latter half of November, the guidelines changed the first of December, but not much for travelers.
I posted a few things in the Belgium forum, but we had a successful trip.
You do need to get a Belgian Digital Covid Pass, your CDC Card will not work. They do issue one to visitors, though it is not the same as the one for Belgian residents, it is only good for 30 days, and will not work outside of Belgium. If interested, I can try to find the link, it can be obtained online, in a very short time. A caveat about it though, it was new when we were there, and about 4 times over the two weeks, someone would scan, and it would reject it, but they would get another worker and scan it, it would be fine. I suspect individuals needed to update their app, or needed to use the Belgian check app.
Most things were open, they did cancel some larger events like Christmas Markets here and there though. Maybe the only frustration was random closures. Some places were just closed, even though their websites said they were open, or hours were changed to a later opening or early closing. It became a running joke as to whether the place we were headed to would be open or not.
Testing was also a frustration. I believe they ask you to test on Day 2 and 7, it is an honor system, no results are provided to authorities (well, if you test positive, that would be reported). Test locations are few, you are told you need to pre-register, but we just showed up, since there was no way we could, as a non-resident, get a prescription code (yeah, yeah, yeah, the website said we could, but then it sends you in an endless loop), and for a basic test, expect to spend ~50 euro. Results take about a day to get, which will cause problems for your return to the US. At the airport, and some Pharmacies, you can get a rapid test, but we were told the cost would be closer to 150 euro per test.
