If you want a town Gisors would be my pick, it's fairly small but with an ancient castle (average, but free) a decent set of restaurants and boulangeries. In that area though, if you're happy with rural you should be going to the Vexin Francais regional park, in particular Villarceaux. I wrote a very short piece on it here:
Skiing, gorgeous gardens, atmospheric châteaux and villages feature among our tipsters’ discoveries, all linked by delightful wine and places to stay
www.theguardian.com
You could do much worse than stay at Chaussy, at Au Relais de Chaussy. Gaelle will make you an organic 3 course dinner for 18 euros! Right on the London-Paris cycle route too.