I loved my trips to Amsterdam, Paris, Munich, Salzburg, and Berlin.
You didn't mention how long you will be in each destination. Your best bet would be to get guide books (I recommend the ones by Rick Steves). That will give you the highlights of each location according to how long you have to spend in each place.
In Amsterdam, go see the Ann Frank house, the red light district, and if you are into art, the Van Gogh Museum and Reichsmuseum (pardon my spelling). Also, there is a sex museum - the main one - that I found interesting. It is not expensive, but has a lot of pornographic artifacts from the 1800's and earlier. Porno started as soon as cameras were invented in the early 1800's. Who knew? The food is not that great in Amsterdam. Native Dutch food is basically fish and cheese. I found a great restaurant called Saturnino at Reguliersdwarsstraat 5 that is reasonably priced and the service is very good. It is also gay friendly. It is not in the tourist area per se (that is Leidseplein), but between there and the train station. In Leidseplein area, there is a Chinese Wok retaurant that I also frequented. It has decently healthy food. If you have time, rent a bike and just ride around for a couple of hours. Bikes are big in the Netherlands - they are everywhere. Be careful when you cross streets. There is usually a bike lane in addition to the street. Don't stand in the bike lane. :-). If you are going to be there a couple of days, get one of the 2 or 3 day tram passes so you can ride as often as you like. You can get these at visitor information just outside the train station. Get a gay Amsterdam map there also.
Paris - well there are the usual - Eiffel Tower, Louve, etc. It will be mobbed with tourists. Go to the Louve early as possible and go to see the Mona Lisa first thing before the hugre crowds get there. Just enjoy the city and relax as best you can. Paris has so much to see and do, you will have to come back.
I loved Munich and Berlin. Berlin of course has LOTS to see. Your best bet is the guidebook to decide. In Munich, go to the large park and eat and drink at the beer garden and listen to the band.
In Salzburg, this is kind of cheesy, but I highly recomend the "Sound of Music" tour. You may be too young to remember or have seen "Sound of Music", but if so, take the tour. It lets you see a lot of the area by bus as well as some of the sights where the movie was made.
I'm too old to go to the gay clubs anymore, but in all these places, there are some good ones I expect (well maybe not Salzburg). But they may start very late compared to here in the US. All these places I mentioned are gay friendly. And the young guys are hot. If you are into gay saunas, these are very good there also. In Europe they have lots of young guys, not like here in the States.
HAVE A GREAT TIME!