It's a decent town for food, but there is no "must" that I know of. The establishment restaurant is Cazaudehore, not bad, good duck confit, has a terrace. La Rotisserie des Loges is also an attractive, simple place. Most of the good Parisian chains are present in Saint Germain (eg Fuxia, Kayser). La Gerbe d'Or is a very nice bakery. There's also a rotisserie next to it (across the street from the movie theater) that is nice. The market on sunday is very nice too. One RER stop away, in Le Vésinet, Avril does über-traditional pastry that I like a lot.