What to do in Haarlem?

Walking around and soaking up the atmosphere of the historic center is probably the most interesting thing to do. Haarlem has many "hofjes" (courtyards surrounded by centuries-old houses), if I remember correctly there is a tourist walking tour around town that includes some of these. See if the VVV (tourist office) can provide you with a route.
If you find yourself stuck for things to do, consider a day trip to the beach at Zandvoort (for a stroll that is, forget about swimming the chilly sea unless you are a masochist), Leiden, Den Haag etc.

Here is what Rick Steves say about the place
http://www.ricksteves.com/plan/destinations/ben/bd-haar.htm

VVV Haarlem

You already mentioned the museums yourself. They have at least two very good ones. Teylers is interesting if you're into history, archeology and biology. The building itself is also amazing.

Het Dolhuys is about the history (and present day) of psychiatry and also very interesting, the displays are fun, interactive and very varied. I don't know if it's all in Dutch or in English too.

I like the city centre and all its terraces, restaurants, cafes.

If you want to dance in a very special location check out the Swingsteesjun, it's a discotheque every Friday, Saturday and Sunday inside a big historical waiting room on a platform in the train station! It attracts a somewhat older crowd (30's and 40's) but young people too, and it's a very happy go lucky (live and let live) atmosphere, everyone's just there to have fun.

If you're interested in visiting Amsterdam (20 minutes by train), have a look at my recommendations for things to see and do there. Go to my website > Travel > Amsterdam Tips.

Veel plezier,
Cécile
Amsterdam

interesting: go to the IKEA shop-beside Spaarnwoude train station-end of bus 5.
cheap breakfast for just 1€-untill 11.00
free coffee if you have their family card-orange-ask someone to borrow
cheap hot meals after 18.00, varies by day/week
and you could see if the things they sell are indeed cheaper in NL as they would be back home.
if its really stormy-go to the sea-Zandvoort or Ijmuiden and see how the waves break-Ijmuiden better-onthe port+pier