We want to leave Paris on train for a day---have already been to Reims. Any other suggestions? Have nevr been to Giverny and wonder if it is overrated?

Giverny is not overrated. It is as beautiful as it is described--but also as overfilled with tourists as you can imagine and becoming moreso by the year. Try to identify a likely day to have the least visitors if you decide to go. The last time we were there (and we've been 3 or 4 times) it wasn't enjoyable, there were so many people. It may need to look at timed entry if they are to retain any of the wonderful feel the basic garden can give when not filled to overflowing with tour groups.

I agree with the above post. Giverny is much easier if you are among the first arrivals of the day. You'll be able to walk aroung fairly freely and see the house without the crowds. Afterward you can have lunch in the small town and then perhaps visit the other museum in town, (I can't remember the name of the museum and the "town" I speak of is one road leading out of the back of the house.)
Another easy day trip but completley different is to visit Chartres. It is only a 30 minute train ride. it has a beautiful medieval old town and of course has the amazing cathedral.

On our last trip we went to Angers in the Loire Valley I think. It has a interesting Chateau (tho not one of the MOST visited ones). We got PREM tickets ahead of time. There are also tours from Paris that visit three of the big chateau but we wanted to go on our own. Another trip we went to Auvers - there is an impressionist museum (NOT with alot of the paintings tho!)We got a bit muddled on the transportation for that trip. Girverny is wonderful but the tourists outnumber the flowers about 10 to 1. Fat Tire Bikes does a nice trip there that lets you skip the lines and has some pleasant biking thru the countryside - if I went to Giverny again, I'd consider that option.

How about London? Around the end of this past March, we did a day trip to London on the eurostar. Left around 7:30 AM and got back around 11:30PM. Had a great time. Did a double-decker bus tour, went to the Victoria and Albert Museum, visited Harrods, had tea and went home!