Please have a read of the RSPCA statement on novelty fish tanks. It's a few years old but it's still very relevant. I can't link directly, but it's the first link on this page.
If you buy a proper fish tank for Christmas, please do a fishless cycle first! Christmas is generally not a good time to get new fish- they can get stressed when in a new home so need a bit of peace and quiet, plus the water quality needs closely monitoring for the first few days and most people are too busy over Christmas.
And if you already have fish, some live food would make a very good Christmas present for them 