Three out of the four Novembers I was there it was gorgeous crisp Autumn weather, sunny and cold. One year we had a heat wave and it was as hot as a good summer in Britain - so you never know!
For good restaurants it has to be Bethesda, (Red line on the Metro), its got the best selection.
My favourites were/are - the Rio Grande on Fairmont Ave (Mexican) and Thyme Square Cafe.
For going out in the evening, most of the tourists head to Georgetown which is fun (take a cab) - the locals go to Adams Morgan (take a taxi there too- the metro stop is nowhere near anything good). or else they go to Dupont Circle.
Dupont Circle is great for little antiquey shops during the day too.
I recommend the Trolley Bus tour as well - most of my guests went on that and said it was worth the money. Make sure you see all the monuments after dark too. They are pretty spectacular.
Old Town Alexandria is worth a visit too if you have time.
If you want a good view of the city The Old Post Office (Federal Triangle metro stop) is pretty high and easier than the Washington Monument.
I could go on - I loved living there and I'm sure you'll have a fanstastic time.