Personally, I think the auto-docking is the coolest thing to watch. Nice bit of automation there.
Other than the standard Rovio battery (which apparently have problems from the word go) I'm going to research what I can fit into the battery compartment. Right now, it's just seven (two 1200MAH's are in parallel, the rest are 2200MAH in series, but these are used/older NIMH's.) AA NIMH rechargables. The battery bay I will need to measure, but I'm pretty sure I can cobble or find something better than the 3000MaH pack that's standard for the Rovio. The compartment I would speculate could hold 10-12 AA's easily, at worst.
Since the Rovio has I2C, I'll now be looking into interfacing to that.. which will mean I'll probably tap the power pack for Arduino power also. A little extra capacity will always be good.
I figure an RC shop is a good idea as to where to look for a good (cheap) NIMH pack.. got any other brilliant ideas I haven't thought of?