Depends on what your channel mainly is about. You could host a small giveaway to see how it goes with a small prize like $10 Psn or Xbox and choose 1 winner.
If the giveaway is going good you can always do a sort like thing later on or host a bigger event.
The more prizes you give away, the more people might focus on watching to get it. Usually, people only stay when prizes are being given away. You could space them out, that way people have a greater chance to look at your work.
Think of it ethically. Only announce it when you hit 300, restrict it to those who were subscribed before 300, then close it and unlist the video when it's over.
Nothing stings more than a person who subscribes for the chance of getting free stuff.
That's a good idea, also maybe if you looking just to give inexspensive things away, gift cards are an easy giveaway. Just ask the commenter or viewer to DM you from the description and then send them the code. It's quick and easy