I was working as au pair in Belgium. A brasilian friend who lived with me and I decided going to Paris two days because we didn't know when we would be so close to Paris again. We took the bus from Brussels and 4 hours after, we arrived to Paris!!
For two days if you want to see lots of thing the best option is taking a turist bus to see everything outside and to get off when you want to visit something.
This is the shortest trip and the best trip I ever done.