The trip from Roosendaal to Brussels and onto Paris is actually a trip I've made. It's not a bad trip. The train from Brussels to Paris is the Thalys, a high-speed train that originates in Amsterdam. It's a nice train, but it is going to eat up over half your day, especially if your trip does not originate in Roosendaal.
You don't have to leave the Netherlands to see cool things. There are a lot of really interesting towns. When I was there I really liked the dichotomy of s'Hertogenbosch and Eindhoven. They are relatively close together, yet one still has its medieval city walls as well as a beautiful Gothic church (was a cathedral before the church started shrinking in the Netherlands) and the other is a modern urban center (mostly because it was heavily bombed in WWII). In s'Hertogenbosch there is also a little boat tour through town on the river that has been rerouted through canals. It's very interesting.
Here is a site about Den Bosch:
http://www.binnendieze.nl/ Click on the British flag for the English-language site.
Also, if you get tired of Amsterdam while you're there, catch the train out to Haarlem. It's a little more authentically Dutch. If you want to go even further, the train runs out to Zandvoort, which is the beach all the urbanites flock to in summer. It's very windy, though, and cold!