I took the 21BOE tour last May 4th. I flew in four days early, staying in a houseboat in Amsterdam for the first 3 nights and then arriving in Haarlem the day before the tour. I extended Paris with an extra three days and then spent 5 days in Scotland. I too entered the scrapbook contest. Here's the link if you're interested: http://thatstevesguytrippedme.weebly.com
I went as a single. I had the time of my life (though I had my share of travel mishaps too, lol)! What took me most by surprise was the cold. I wish I had packed more warm clothes! I had one pair of jeans, a pair of black slacks, a few pair of capri pants, a bunch of lightweight shortsleeved shirts and two button up sweaters. I wish I had packed fewer capris and at least one more pair of jeans. I wish I had a hoodie jacket/sweatshirt/pullover sweater, a couple of long-sleeved tees and a couple less short sleeved things. Be sure to tuck in a pair of gloves and a hat if you plan to take the gondolas up to the Schilthorn in Switzerland!
As far as sites to see? So many! It really depends on your interest level. I have really enjoyed my mini Viator tours. If your BOE tour is going to Keukenhof, then perhaps take a day tour to see the windmills/Volendam, take a mini tour to Delft and watch how they make pottery. With my pre-BOE time I took a day trip to Bruges (I'd been to Amsterdam 3 years before that and seen a lot of the city, so wanted to experience Belguim). Visit the Viator pages. Even if you don't take their tours, it can give you great ideas of what you might want to see and do. Of course Amsterdam has tons of great museums, but you'll see a ton of those on your BOE. It really does depend on what you like to do. I took a canal cruise, even though you get one on your tour. I was so tired when we took our canal cruise on the BOE that I could hardly stay awake to appreciate it. So I was glad I took one on my own.
Like the other posters, give yourself plenty of time in Haarlem too. It's a small town, but has so much to offer! I made a point of reading the Diary of Anne Frank and The Hiding Place- Corrie Ten Boom right before my tour. It makes it all the more special then when you go to their homes.
Have a great time on your trip! I can't wait to go back!