I've been to The Netherlands twice now, and it's one of my favorite countries on earth. I just love it.
First off, I have found the Dutch people to be some of the friendliest in the world (and I've traveled quite extensively too). First time I was there was with my family and the second time was with friends. I remember us being lost somewhere in Amsterdam and stopping on the side of the road with our map out and several (not just one) people going out of their way to stop and help us and point us in the right direction. In addition, I also found the people to be very chatty and just all around pleasant in general. I honestly don't think I had one bad personal encounter anywhere in The Netherlands.
Moving on to actual cities/places/things to do. Amsterdam is a great city. I know that it has a bit of a reputation as being a place to party because of its Red Light District and the legalized prostitution and pot, etc. However, while this is true if you are into that scene, beyond that there is SO MUCH more that the city has to offer. Amsterdam is a beautiful and charming city. The architecture and the canals are gorgeous and one of my favorite things was just to wander aimlessly around looking at the shops and just taking it easy. Jordaan is a great district and one of my favorites with lots of great shops and restaurants. It's great for an afternoon of strolling and shopping. Besides that, there's tons of museums (Van Gogh, Rembrandt, Rijksmuseum, etc.), the IAMsterdam sign is fun for a photo opportunity, Dam Square and the Royal Palace, and of course the Anne Frank House which is amazing and well worth the wait (get there when it opens to beat the crowds). The river cruises are great too. We did an hour long one and it was relaxing and informative. Plus, it's nice just to get off your feet. There are also a lot of great parks too. Vondelpark comes to mind. Those are things that just first popped into my mind, there's tons more stuff to do as well. Like I said previously, there's also the Red Light District and the party scene if you're into all that.
Outside of Amsterdam, I've also been to Rotterdam (I have a Dutch friend who lives there) and Maastricht. Both of these cities are very nice as well. I especially like Maastricht. It's a very charming city with a lot of character. I've only been to both of these cities for a day each though, so my knowledge is much more focused on Amsterdam. If you guys have time, I think a day trip or two would be fun. You can look for day packages online with different companies. If you do go to Maastricht (either with a tour group of just by hopping on a train at Amsterdam Centraal one morning), then you're right on the borders with Belgium and Germany too, so you can hop over if you wanted to.
As far as places to stay, there are tons of hotels ranging from all price ranges and star ratings so I'm sure that you'll be able to secure something if you search. Just search sites like Orbitz, Priceline, etc. and see what pops up. Sometimes I get a nice 4* hotel for like $80 a night in the off-season in different places. Just play around with it and you can find a good deal. The public transportation in Amsterdam is also very good. The trolley system is great. It's easy to use, relatively cheap (compared to say the Tube in London, for example), and clean and efficient. There are attendants who sit in these little cubicles (for lack of a better word) and you can go up to them and buy your ticket when you first hop on. Tip: Remember that you have to scan your card when you get both on and off the trolley. Besides the trolleys, we just walked everywhere. The city is very walkable and quite compact. You'll learn your way around very quickly. Also, bikes are the main form of transportation in Amsterdam. They're EVERYWHERE. You can't miss them. I think one of the guides told us that there are more bikes than people in Amsterdam. There's literally millions of them. You'll see what I'm talking about when you arrive. It's fun to rent one and ride around for a few hours too.
The food is also good in my experiences. There are a lot of Italian, Indonesian, Chinese, and Argentinian restaurants, if memory serves. Of course, you can get basically any kind of cuisine, but these really stick out to me. Another great thing is that there are tons of fry stands and little shops everywhere throughout Amsterdam. You can stop and pay like a few euro and they give you a massive portion of fries. They're delicious. I also loved stopping into bakeries because the Dutch pastries and sweets are to die for. I remember specifically I would always get this Dutch thing where it was basically a ball of fried dough with sugar sprinkled everywhere on it. It think they're called Kwarkbollen or something like that. Sounds weird, but try them. Again, delicious.
As far as language, I find the Dutch language very hard to pronounce personally and a little strange (no offense Dutch people!), but literally everyone speaks English (their school system and language skills are superb). People will address you and speak to you in English when they see that you're a tourist. However, I always learn basic phrases when I travel to different countries and I found that the Dutch people really appreciated me attempting to use their language (not very successfully though lol). The country is also very accepting and fun and they have a strong live and let live attitude. I love this about the Dutch and I find it very appealing.
Also, almost forgot, if you're there in May then go and see the tulips! They should be in full bloom I'd imagine, so go see the famous Dutch tulips.
So, yeah, that's a long post and I'm sure that some of our Dutch posters will be able to give you more/better advice, but that's just some of the stuff that I remember. Like I said, I truly love Amsterdam and The Netherlands in general. I'm sure you'll have a great trip! Enjoy and come back and let us know about your trip!