Everything. Your tariff will not cover roaming.
I think there's an EU agreement on maximum text and call charges. I was pleased to find (on Orange) that when I was in Spain, texts back to the UK only cost me 10p each. I didn't use it for calls.
Unless you want to be charged a fortune for data usage, disable cellular data completely before you leave the country. You won't be able to do anything that requires data, but the alternative is to pay through the nose for it.
