I'd like to make a Peggy that:
-powers on when the sun goes down(need a light sensor)
-runs off rechargeable batteries(which kind is best for this app?)
-charges itself back up during the day.(what's the *smart* way of going about this?)

I've done some research on each task but i've found that practical, firsthand experience advice is worth tons more than documentation.

Any advice is very appreciated.

Well... that wasn't one of our design goals, so you'll need to be a bit careful how you go about it. There are plenty of "solar garden light" circuits out there; some of them run on 4.8 V (corresponding to 4 NiMH cells in series); it could be enough to run a Peggy if you use a large and/or efficient solar panel and/or high capacity NiMHs.

The most simple solar garden light would take the output of the solar cell and put it *through a low-leakage diode* into the battery, with a zener clamp to prevent the output voltage from exceeding 5 V to the Peggy.

You don't really need a separate "dark detector" circuit to switch the power to the Peggy-- you could just leave the battery hooked up to it all the time and use one of the analog inputs on the Peggy along with a CdS photoresistor to detect when it has gotten dark, at which point you turn the LEDs on.