Well you have a great start with this idea! Gamifying the donation's process as an incentive is the way to go and providing a lottery on top of an address  is simple and elegant. But to be more "socially engaging" the users must have an incentive to "collaborate" to achieve some goals and so sharing the offer as much as possible. For this i think a great way is to provide the donation process as a mean to unlock the access of a bonus content that is unlocked when a threshold amount is reached. This can be as simple as a widget with a progress-bar and a qr-code to accept donations, when the goal is reached the progress-bar is replaced with the link of the desired content.