I think this is a great idea. APK has helped me, and Madpk, SO much, I have often wondered how to give back. The Madison Jam literally could not have happened without APK's help.
So, with the PayPal donations button, are you abandoning the idea of a Premium Membership? Or are both options still available?
Here are my suggestions for what a premium membership could include. Feel free to add/delete/modify/laugh:
0. no ads
1. subscription to the newsletter (at no charge; included in cost of membership)
2. entry into a yearly drawing for APK merchandise (you draw, say, 5 winners, or maybe a number of winners equal to some percentage of the total premium membership--winners get one of the following items: water bottle, t-shirt, hoodie, etc. Just one prize per winner)
3. a birthday card (sent via actual snail-mail), signed by the Tribe, on their birthday (kind of cheesy, but I'd dig it. Plus you could make blank parkour themed notecards for sale, and use those to send to people as well. Of course, it would require someone to manage the sending out of cards, but much of this can be automated and all it would take would be for someone to ensure that the cards got signed and mailed out)
4. Perhaps if, in the future, APK did "big ticket" type contests where someone could win something like a free weekend of training with a Tribe member, or at Primal; Premium Members could be automatically entered into the drawing without having to actually go and fill out an entry. If they decided to fill out an entry form, that entry would stack on top of their automatic one.
5. Access to certain aspects of the forum, although at this point I'm not sure what those would be.
I dunno. These are just my thoughts.

I hope they're helpful!