The two main areas for culinary activity are around a night market that fronts onto the river and also along Khatha Ton Road, near the Tesco supermarket -- we spent all our time down around the river as Khatha Ton Road is a long walk from the river.
For breakfast try along the parallel running Sap Siri and On Chan roads (they run between Kraipetch and Ammarin roads) for a selection of curries and other Thai breakfasts, while on the corner of Ammarin and Kon Thong roads you'll find some yummy deep-fried spring rolls for a post-breakfast munch.
On the south side of Kraipetch, between the Araya Hotel and Rot Fai Road, are a trio of hole-in-the wall noodle and rice joints -- we found these to be good for breakfast and lunch.
Down on the river, opposite the National Museum, you'll find D-Kunst Gallery and Cafe. Housed in a beautiful wooden building, drop by for a coffee and take a look at the exhibits while you are there.
For dinner, the covered night market (also open for lunch) is right by the river (just look for the clocktower) and offers a standard collection of typical Thai market fare. We had an excellent baked salted fish here and also on the southwest corner there is a little cafe that does syrupy, strong coffee.
If you're after something fancier, Mai Thai serves up some of the best fare in Ratchaburi, with a good variety of Thai, Chinese and other Asian dishes available. The somewhat gaudy interior design is like nothing we've seen before. Prices are moderate. You'll find it on the soi to the east of the Numsin Hotel, running north to the river. Open for lunch and dinner, Mai Thai also has extremely fast free WiFi.
While we didn't try them, when we got lost wandering we stumbled upon a couple of floating restaurants on the river to the west of town -- could be a comfortable way to while away a late afternoon or evening.