I agree with what others have written - focus on the milestones: 1,000, 5,000, 10,000, etc... I just did my first one at 1,000 subs which was a $20 gift card for the winner however to ensure that it was a dedicated subscriber that won, I didn't mention anything about the giveaway in the title of the video - just called it "Thank You For 1,000 Subscribers!" Anyone who watched most of the video (and it was a short one, only about 3 minutes) would learn that I was doing a giveaway and just had to do something specific in the comments. That way it was primarily those who were subscribed to me who were going to see it and comment.
I have seen a lot of challenges and QandA which is i think one of the most requested videos! When i will get to 100 subs i will do a QandQ i think!
If I get to 1k subs by March, like I hope, I'll draw some Studio Ghibli fan art + Yiynova 19 giveaway for just that.
Many challenges are done for milestones, and that is a pretty great idea, i even love it sometimes :) I have around 100 subs, and i plan to improve my content more and more, but when i reach 500 subs i would probably do something xD