buy him something practical, but small (some new bibs or rubber duck for his bath etc) and wrap it up so that he has something under the tree from mummy and daddy. He will be spoilt rotten from friends/family anyway, so seems silly to spend money on stuff at that moment.
having said that, if you get toys for him, he will soon grow into them so it won't be a waste of money.
i really like the idea of the bauble on the tree, decorate it with his initials and 'babir first birthday' or something similar. he will love looking at this as he gets older and children tend to remember these special tokens of their childhood.
more importantly, just relax and enjoy your first christmas with your new baby, it's such a special time and will probably be the most peacful christmas you will ever have as parents.
for the next five years, your liing room will be filled with the sound of fisher price toys blinking and flashing away at you, few years after that it will be celebrated to sound of talking toys with american accents, then you are on to the playstation years
believe me, you should make the most of this christmas!