One thing to consider is that the Kindle Fire comes with a free month of Amazon Prime (free Prime-eligible movies, one book borrow per month, and free two-day shipping) for customers who have not used Prime instant videos over the previous year. This free trial period begins automatically when you first activate
the Kindle Fire, so if you do that before Christmas, the Prime countdown clock will have started ticking. If that matters to you, you may want to wait and activate just a few days before the gift is given.
More info: Amazon Prime and Kindle Fire