Have a fabulous time in Krakow, Helen. It really is a beautiful city. Try and learn 'thank you', hello and goodbye (difficult I thought) before you go and the Polish will be impressed and make life even more easier for you. We can't wait to go back.
Back to half term, how about Barcelona? It's cheaper than London, friendlier than London and you can get decent accommodation for a reasonable price. Plus it warm still in October, always a bonus.