So, here is what I think
Generally I wouldn't recommend giving any gifts unless it's something that they really want. If you want to give something, just give them some cash or a gift card. Or you could do them a favor, help them with an errand etc.
It is quite possible to get an INTJ a gift they will be very happy with.
I agree that you should get them something they want. They will not want you to do them a favor or run an errand.
As far as what they want - it isn't that hard to figure out if you pay attention to what they buy themselves.
The only time ive ever given a "good" gift for my INTJ dad was when I bought him the same thing he intended to buy but a week before he got around to it.
Smashing idea.
In summary:
- it should be practical
- it should be something they already want
- one good quality thing is better than ten mediocre quality things
- cool new technology is ideal but it must be practical
- don't buy them a book; you'll never be able to guess which ones they would be interested in
- whatever you do, don't get them a gift card
- clothes are actually ok but not terribly exciting
So, some examples would be:
- 10 pairs of black socks - all the same kind, so they never have to worry about matching them; they must be soft and comfortable
- An iPad
- New underwear
- A Blue Ray player
- A Porsche convertible
- New CD from their favorite artist
One of the best gifts I ever got was a GPS for the car. It was cool and immensely practical.