I'll be stopping in Brussels for a couple of hours on trip from Amsterdam to Paris. Is there a place to store my luggage -- counter or locker -- while I wander a bit?

There are lockers.

And attended luggage. If you have a large suitcase, the attended luggage will work better as the lockers that hold full size suitcases are virtually always full.
Both lockers and the attended luggage area are located in the new wing where the Thalys/TGV/Eurostar trains run.
The price for attended luggage is about 3.5€ per piece. If you have only one piece and it's fairly large, attended works well. If you have more than one piece and they're small, then lockers are probably a better value.

Thank you.

Good to know. I'll be doing the same thing on a trip from Koln to Paris. Thanks!