Hmmmm. Watford is not exactly a culinary destination.
There is a crop of new restaurants at the near-bottom end of the High Street - three in a row in a lovely white art-deco-ish building, of which one is a Carluccio's. You generally have all the family-friendly chain options in that area - Pizza Express, Zizzi, Ask, Prezzo. A little further down there is a Lebanese restaurant which I have been to and thought was very good. At the very top of the high street it is pedestrianised and has a La Tasca, Nando's etc. Oh and Yates's Wine Lodge.
My next recommendation would be a South Indian (i.e. mostly vegetarian) restaurant on Merton Road quite near the centre, which google maps tells me is called Rathiga. Parking is a bit crap around there but you might walk easily if you're staying near the centre.
If you need to eat on a Tuesday, Friday or Saturday lunchtime, go to the Charter Place Market. There is an utterly excellent sushi/noodle bar there, also a very good Indian cafe (I think these two share seating), and smaller counters for Caribbean and African hot food.