Brussels, Amsterdam, to Den Haag

Take the train. See the Dutch rail site at
www.ns.nl for schedule and costs. No reservation required or allowed.

You might want a 24-hour ticket, if you will be visiting other places besides den Haag within the specified time period. For example, I think if you were visiting den Haag that day, and then going to the airport the next morning, the 24-hour ticket could pay off. Or if you are coming back from den Haag, and want to stop somewhere else, you can do that with the 24-hour ticket.

When I was last in Amsterdam, I remember I had to pay for my tickets from the machines with coins, because my non-chip credit card would not work for that, and neither did bills for some reason. Lining up to get a ticket takes a long time typically. So just have lots of coins ready.

Thank you very much

I concur with WillTravel on the coin vs. credit card vs. cash at the train stations. I found last April that I had to pay cash at the window --- machines only accepted coins or credit cards with PINS. There is a slight charge (0.5 Euro) for purchases at the ticket window. Lines can be long in Amsterdam central, so plan accordingly. Do check with the information booth regarding discount tickets. There was a Spring special for a one-day all-the-trains-you-can-ride special, but only for an initial departure after 9AM (no morning rush hour travel). Other special tickets should be available too.

I am not sure the 24 hour ticket works on the trains, only on buses trams and metro. They haven't fully implemented the ov chipcard on trains yet. This is something you must check at a the service desk - it needs to be validated for train travel in any case.
Do not try to check this on New Years Day - do it before then. There may well only be one window open on NYD. You can buy a train ticket ahead of time and stamp it yourself on the day of travel at the little yellow box on the platform.

I have checked - the 24/48 hour tickets are only valid for the buses trams and metro of the company from which you buy the ticket - so only in Amsterdam on the GVB buses etc. If you want to use one in Den Haag then you have to get one in Den Haag. You can buy them from the bus driver.
You cannot use them on the train nor can you use one you bought in Amsterdam in Den Haag.

You would need to buy an anonymous ov-chipcard and load a lot of money on it to use it on all public transport. NS takes €20 off when you check in and then repays the difference when you check out. If you don't check out you lose the €20. If you don't have €20 on the card you cannot check in.

So buy a normal return train ticket, undated ahead of time so you can stamp it on NYD and travel. Buy your bus tickets on the bus. It will cost a tad more but is the easiest.