i went camping for the first time at AI last weekend. I called the ranger Thursday night regarding the camp sites and he suggest that I come early on Friday. He said if I come at night on Friday, the drive in camp sites will most likely be full. I arrived Friday night at 11 and a lot of the spots were available.
This time of the year, its first come first serve but you should not have any problem getting a spot. I stayed there for 3 nights and it was $46 for camping fees and car entrance fee. I definitely recommend the drive in vs walk in, you can park next to your tent and walk a very short distance with your cooler + tackle and fish for the whole day. feel free to pm me if you have questions.