Paris is 35-40 minutes away on the RER. It's really easy to get there.
If you look in The Very Useful Info sticky there are details on it, including links giving prices of different tickets. We bought just straightforward tickets, one train in, one train out, but for a little more you can buy a Mobilis ticket which will also allow you to travel round Paris on the Metro that day. I think the cost for a one day Mobilis is approx 12.