I've done the Old Town and the Jewish Quarter tours with Free Walkative Tour I can't recommend it more.
If you spend at least one day in Krakow and don't join one of these tours, you're definitely doing it wrong.
The tours are very informative and fun. You learn about the history and myths of the places. You get to discover some off the beaten path attractions and really get to know the city.
If you're worried about the "physical" strain of the tour ... don't. It's really not that hard and it's never done at a "high pace".
I have to leave a recommendation to Nika and Chris. My tour guides were awesome. Friendly, funny, and knowledgeable.
One last thing ... Do leave them a good tip. They deserve it and you'll enjoy your visit so much more.More