Train Amsterdam to Delft

First you don't buy RT tickets. You buy one way tickets each way but sometimes they might have a promotion which includes the return for a special price. Second, you can do that provided you are not riding a train that requires a seat reservation. Generally local and IC trains have mixed seating - reserved and non reserved. So just look on the schedule and see if a reservation is required and don't use that train.

"...Generally local and IC trains have mixed seating - reserved and non reserved...." You can't make reservations on domestic Dutch Rail trains. All seats are first come first serve.

I was using www.ns.nl and looking at their Same Day Return journey fare (what I was calling RT) from Amsterdam to Delft. We would buy 2nd class tickets and just buy them at the train station. I'm just wondering if when purchasing them we needed to indicate that we wanted to get off the train in either Haarlem and/or Den Haag, or if we could just decide last minute that we wanted to stop in either town and and then get on a later train to continue our trip to Delft using the tickets we'd already purchased in Amsterdam.

So long as these are open tickets, good for any train and limited only by the return being within the same day, and not for a specific train number, you can get off and on at any time.

Thanks! It will be nice to stop along the way.