I've been to U modre ruze and it was very good, but do note that the water was insanely expensive (about 8 euros for a litre!). The rest of the beverages, as well as the food, were quite reasonably priced, so better get some extra wine. Creperie U slepé kočičky on Milady Horákove is a fabulous place for a light and/or informal meal, while U Zlateho Stromu on Karlova offers a very good restaurant (though it's not totally formal, either, so don't go there expecting a multi-person team standing at the edge of your table). I've also had great experiences in a number of small restaurants and pubs whose names I cannot remember for the life of me