Poselska is one of the nicer, more romantic streets in the Old Town - just a step away from Grodzka lies this cobbled, underlit and a bit twisted street. But it's also quite rich in restaurants - just in a few steps you've a choice of a Hungarian, Corsican, Mexican, and Italian restaurants, followed by an Irish pub. (And, what's best, most of these are really decent - or better.)
"Trzy papryczki" is one of the better pizza places in town. It's not big - just a few tables inside - in a very nice rustic room. What's even better is the garden in the courtyard, that's really cosy and makes you feel almost like you were in Italy. Especially in summer - the inside room is not air-conditioned and it's really difficult to stand the heat.
As for the food, I was never disappointed with pizza here - it's simply great. Even though the oven's not burning wood in it, they're great (they tend to have the thin, crispy base rather than the thicker one most popular in Poland). Some of their specialties were the quatro formaggi and the Parma ham ones. Their namesake pizza, with grilled peppers and anchois is also one of the best. The choice of other dishes is not very big and frankly, when it comes to meats, there are a few better Italian places in Krakow (one of them sits just next door - its sister establishment "Corleone") - so if you're feeling especially carnivorous, head there. Thankfully, they share the same dessert menu - I would recommend the lemon pie, but what's also very good is their ice-cream with pepper (yes, that's black pepper) - a very nice combination.
The prices are pretty much the same compared to the better pizzerias in the area of Old Town - from 20-30 zl per large pizza. And after a pizza, it's only one step away from the very romantic Kanonicza street - even more recommended.