If you don't mind the drive, go to Longwood Gardens in Pennsylania. It's worth the price and time- spend the whole day.

If the weather is nice, the evergreen collection and the bonsai museum at the National Arboretum are both nice.

If it's really cold and nasty and you need a "pick me up", all winter long I suggest people go to the rain forest exhibit at the National Zoo.

The rest of your ideas will probably take up most of your "spare" time.

In case folks haven't heard about it, the USBG has a special exhibit now with Washington buildings and monuments made out of plants. (It's not on their website yet, but I saw it on the news.)

thanks for the comments. hadn't thought of the zoo as a place to hit. (I've never been....)

Longwood is out this time, because this friend is coming from NYC - it's half way home, lol.

The Bishop's Garden at the National Cathedral is a nice place to visit, even if it will not be looking at its best.
This might not the top of your wish list, if the weather is crummy.

Hope that helps,

Dan