As a Dutch person, I don't think many routes are really scenic as the country is predominantly very flat. Personally, I like the route between Maastricht and Heerlen the most as it's very hilly over there and very different to the rest of the country (and very far from Amsterdam). Also Maastricht - Sittard is a nice, curvy route through the hills of Limburg. For a typical polder landscape, many examples exist in the west of the country, I like Utrecht - Gouda - Rotterdam the most for that. Also some railway interest on that route with some junctions, branch lines, the large stations of Utrecht and Rotterdam etc.
Another personal favourite is Zutphen - Arnhem because of the typical Dutch river landscape on one side and the start of the hilly Veluwe woods on the other side. It also has old manor houses on the hills next to the railway. Also I like Arnhem because it's very hilly around the station and the landscape changes dramatically if travelling through to Nijmegen.
If your interest is in obscure rolling stock, you'd need to travel to the border I think. The Belgian cross-border local services use very old stock. Other possibility is to use the IC Amsterdam - Berlin within the Netherlands. This uses German coaches and a Dutch locomotive as far as the border in Bad Bentheim. In Bad Bentheim the train is stopped for 15 minutes to change locomotive and voltage on the overhead wires. It all feels very outdated in 2023 and will not be around for many more years I believe. Within the Netherlands, this train is just a normal intercity so no special tickets required.
Buying tickets is easiest and cheapest on the website or in the app. I think it should accept all credit cards now, but not sure as Dutch people don't really use credit cards domestically. Buying at a TVM or ticket office will attract a surcharge. If you travel a lot, some kind of Holland Travel Ticket or Amsterdam Travel Ticket might be useful. The former is a national day rover for all public transport in the Netherlands (despite that Holland is in the name) and is €48 off-peak:
https://www.ns.nl/producten/en/onbeperkt-reizen/p/holland-travel-ticket
If you have any question, just let it know