How bad will the tourist throngs be in May in these cities? Is this still considered shoulder season and not quite as bad as summer would be? I was in Paris once before in February, so I'm curious how much different the crowds will be this time around. Thanks for any advice!
It's shoulder season; crowds wont' be bad at all. Paris is always busy, but not as bad as high summer. I did Paris, Bruges, Amsterdam during May a couple of years ago, and it worked out great. Had Bruges almost to myself...as it's on the way, it's something to consider. Weekends in Amsterdam are always crowded year round. If you can, try to time it so you will be there mid-week. Unlike most other cities, hotel rates in Amsterdam are cheaper mid-week as opposed to weekend most everywhere else.
On your list, I've only been to Brussels in May. Even in the height of summer, the only area that really feels "crowded" with tourists is the pavement directly in front of Mannekin Pis. Its a busy city any time of the year, but it never feels like it's overrun with travelers. Certainly not in May.
I've hit all three in May. No problems.
Have been to Paris in May (only visited Brussels in July) and loved it. Mild, balmy weather, and fewer shuffling hordes of tourists.