I just plugged your dates into hotels.com and quite a few came up in the 3 star range. This is where I generally shop when traveling solo...although it also depends on reviews so I might go for less if the reviews were really good. (My next solo stay in Amsterdam is at the Hotel Fita if you want to judge me). As you are with several people, you could lessen your stars and stay for less. My requirement is clean and safe. Safe is not as big a factor when I'm not traveling alone. So you can have a hotel downtown at relatively last minute, but you might have to give up some services if you are accustomed to staying at 3 to 4 star hotels. A 2 star hotel can often just mean there is no elevator, which hardly affects cleanliness, service or safety. Check out hotels.com and booking.com. Include TripAdvisor search for reviews there as well. Then you can then reserve your rooms direct if you prefer.
PS. We stayed in Harlem for a night on the way out of the Netherlands on our last stay. It's a cute little town, but has enough to more than fill one day and I hate taking a train into the location that I really want to see. I was listening to RS on that one, and it was OK, but we preferred staying in Amsterdam, which we did upon arrival.