We've never used dollars in Poland, only zlotys, although once we stayed at a private B&B (ie someone's relative who takes paying guests) who preferred her payment in pounds sterling.
There has been a thread on Poland/Krakow before, have a search on the archived threads.
Hints and tips? Really depends what kind of trip you had in mind. If you're going without kids, Auschwitz is a must. It's cheapest to take the train (around 50 mins journey), and there's a bus stop just outside the train station that takes you there.
For food, you will find everything you need in with regards to resaturants in the main square. For lunch, grab a kebab or a large slice of pizza for 6zl (about £1). Take a wander up to Florianska Gate - wonderful artwork for sale hung up on the old city walls. If you're lucky, you'll hear a truly extraordinary accordianist busking there. For a fantastic coffee, go to Jama Michalika on Florianska street, if you don't have a nut allergy, order the 'tort' to go with your coffee.
For dinner, I would recommend the restaurant under the Ratusz (Town hall tower) - it's a gothic dungeon if you go down the steps, but in the summer they put tables outside (as does the rest of the square). They serve nice salads.
For nice traditional polish food in an elegant atmosphere, there's the restaurant 'Pod Gruszka' (under the sign of the pear) just off the square. Listen to live trad jazz (for free!) on a tuesday and wednesday night in the Harris Piano Bar on the main square, where they also serve coffee, as well as cheap alcohol (2 glasses of beer cost us 13zl last time). Loads of other nightclubs dotted around the square if your tastes are more contemporary - just wander around and listen out for what appeals.
If you're brave enough to use buses, the fantastic Aquapark is a bus ride away. They have great toddler facilities, and amazing tube slides for older kids (and adults) Ask for details at the tourist information place in the arches on the main square.