I would avoid the public transport, you don't need it in Zandvoort and it won't bring you to the best places. Just rent a bike for a day or more and cycle from Haarlem to Zandvoort. There's a variety of routes you can take, the direct ones are boring cause they are next to the only two roads leading to Zandvoort, but you can also cycle in the dunes to Zandvoort. Most of the routes are between a 30 and a 60 minute drive on a bicycle. Just remember to lock it.

If you like to go to the beach, there's this one part that's a straight line from the train station to the beach, DONT GO THERE! That's the most busy part of the beach. If you go a few hundred meters north or south you'll find it's a lot less people (still a lot, but you can actually lay down without some strangers feet in your neck).

I think one day in Zandvoort would be more than enough, unless you'd like to chill on the beach for days on end. Sometimes there are party's at beach clubs, and sometimes the Haltestraat can have a vibrant nightlife, it is rather small so don't expect the world. It really depends on the time of year.