Something I always wanted to do was ride the TGV, so when I went to Paris I decided to make a day trip via the TGV someplace. After much discussions with my travel buddies, we decided to take the train to Brussels. So I was killing 2 birds with one stone; getting to ride the TGV, and seeing another city/country!
Unfortunately the actual TGV train was a little more expensive than I had hoped. So we ended up on the Thalys, which is basically the same type of train, just a different company http://www.thalys.com/
It's about a 90 minute ride to Brussels from the Gare du Nord station in Paris, and what an amazing ride it was! Aside from planes, I've never been on anything so fast! And the sounds rushing by were great! It's so much easier to do the whole train thing too as opposed to flying, because you don't have the whole security issues to deal with compared to airports and such. Plus trains pretty much always run on schedule. Usually that is.
Brussels was a fun day trip. We arrived, took the little trolly/subway the city has to the city center, and immediately went and got Belgian waffles. I'm not a big waffle fan, but let me tell you, the waffles there are to DIE for. And when you see places in the States selling Belgian waffles, that is not what they look like there. Over there they are much larger, and oh so much better.
We strolled around the city center, taking in the views and scents. The smells of chocolatiers abound so of course we bought some in a nice fancy chocolate store. Best chocolate ever!
Of course, a trip to Brussels must include a stop at the Mannequin Pis (that's the famous peeing boy statue). http://en.wikipedia.org/wiki/Manneken_Pis It is TINY! I don't know why I thought it would be bigger, but it's not. And it's located on this inconspicuous street corner that if you weren't paying attention, you'd probably miss. Anyway, I saw it, don't need to go back.
Had lunch at a local brewery (again another thing Belgium is famous for - beer). Spent a while eating, drinking and watching some local futbol match. After some shopping, it was back on the Thalys for Paris. 90 minutes later, we were back at Gare du Nord.
So, as a suggestion, if you have the time when you visit Paris and want a day trip, or even an overnight trip, take the train. Go to Lyon. Or London. Or Brussels. Or anywhere else the trains go. It's a lot of fun to see other places than what you are used to in your vacation area.