Alkmaar and Hoorn

Hoorn is certainly worth a visit. In its days it was an important and rich port town, even one of the founders of the Dutch East India Company, which still shows. I can recommend a visit to the Westfries Museum.

Agree with tonfromleiden. I visited Hoorn for a couple of hours and enjoyed it a lot. And I was there on a Monday, when the museum was closed. And it rained.

I combined it with a longer visit to Edam. (By the time I got there the sun came out.) I had grand plans to visit a whole number of the localities nearby (Volendam, Marken, Monnickendam), but once I'd spent most of the morning and afternoon in those two places, I was ready to return to Amsterdam. So if you're like me, don't schedule too much into one day. Alkmaar and Hoorn should work out fine.

I traveled by bus (http://www.localbus.nl/), but it doesn't look like they service Alkmaar, so you are probably better off taking the train.

From Alkmaar the train is indeed the only realistic option. Half-hourly direct connection, taking less than 30 mins. From Hoorn back to Haarlem can be done in 45 mins, but that involves a change at the somewhat confusing station of Amsterdam Sloterdijk. It's better to take the slower, but direct train to Haarlem. Use this train planner.

Hoorn is a great seaside town and the Westfries Museum is spectacular if you love the Dutch Golden Age. Try to book the museum's "virtual reality" tour of Hoorn in advance. When I visited, the showing was sold out at the time I wanted to see it.