Unfortunately, I have no experience with booking tour there. I try to ask around some of my friends about it and get back to you. The only time I visited Auschwitz it was a high school trip and my involvement ended on handing my money to someone. :/
I think that you should be fine with booking for about a month in advance. That all said, from what I know most museums are OK with you tagging along some other group if the money in the end will check out.
Kraków will blow your mind with its Old Town if that's the case (same goes for Prague). I can compile you a good list of places to visit over the weekend and reply along with update on Auschwitz.
As an insider I have some trouble with pin-pointing what would be the best choice, so until I'll give it a bit more thought I am going to just focus on general tips on getting around Poland.
1. Learn at least a few basic phrases in Polish.
You can quite comfortably talk in English with almost everyone under thirty, and most people are going to be rather friendly anyway, but it's a type of extra mile that goes a long way.
Excuse me. - Przepraszam.
I'm lost. - Zgubiłem się.
Can you help me? - Czy możesz mi pomóc?
Do you speak English? - Czy mówisz po Angielsku?
Thank you. - Dziękuję.
BAM! That's all you need to know to win people over. ;)
2. We are big on public transportation.
You can go almost anywhere by bus, tram or railway. Get this app (also available for iPhone) and use it. It's a life-saver and beats even the commute you will get at ZTM (Zarząd Transportu Miejskiego - Office of Public Transportation) websites.
Prepare for fat and a lot of pork. I don't know if it applies to you, but you are unlikely to get anything kosher. Lard, pork, sauerkraut, potatoes and thick soups (sour rye soup is a good example, along various types of borscht. We also have many soups based on milk) are quintessential to Polish cuisine. You will likely get it in a bit downplayed style in most restaurants, but you should know what's the general style here.
4. Be on your guard.
Poles are, in general, friendly. But there are arseholes who will deliberately direct you the wrong way or try to steal your stuff. That same thing applies anywhere, really. Germany, Czech Republic, Latvia… there are thieves who will use and abuse the amount of tourists for own gain. It's not really a tip about culture, but a generally a good advice. Emergency services in Poland are accessible via mobile phone at 112.
If you have any questions, feel free to ask. And please excuse if this whole post is jumbled, I had to do a lot of crap between each paragraph and might have repeat myself or write out of order.