My normal routine when I travel alone in Europe is to have breakfast at the hotel if it's included in the rate, lunch from a grocery store (unless I'm someplace like Vienna or Bavaria where I'll go to a wurst stand), then a larger sit down dinner and an adult beverage at night. But I'm ok spending US$10 on lunch and US$25 on dinner if I'm alone, so if your budget is less you might need to scale back a little.

Still, grocery store lunches are great, because they help get you immersed into the local culture. I love just walking the aisles and seeing what's different. In some countries, the big groceries are in department stores, such as El Corte Ingles in Spain. In others, there are free standing groceries.

Frommers $X/day books have decent restaurant recommendations if you pick the "splurge" places (which really aren't much of a splurge). My restaurant rule when picking from basically random restaurants: For places that are at least 1/3 full, maximize [(% full of locals) - (% full of tourists)].