Be cautious with your promises. Many an ambitious game has faced understandable delays and suffered ridicule for it. The biggest thing a developer can do is keep the fanbase informed, and be honest when it comes to promised rewards. "Alpha access in January" sounds good, but "Alpha access when the game reaches rev 0.2" is safer.
As for specific products, how about livecam or streaming footage in exchange for donations? For instance, every time fans donate $5000, you could do a 24hour livestream of you coding and testing (Value could change to ensure that this sort of thing only happens once a month or however often...).