Give them your heir.
Right now, in the beginning those lands will be pretty worthless due to the conquest penalties, removing all levies and tax income. (The penalty for a different religion last the longest, so try to convert the counties asap) However, several counties in the Middle East, like Alexandria and Cario, are very well developed with many holdings and once the penalties are removed they bring in a lot of cash and troops. So they're definitely worth holding on to. Giving them to your heir keeps the lands in the family until the penalties are removed.
Do keep in mind that you're surrounded by Muslims and there will be countless attempts to retake those lands from you. Luckily, you'll able to hire Holy Orders without paying any upkeep (you only have to pay peity to hire them). Which, if no one else hired them, gives you thousands of heavy cavalry to crush the heathens.