Adafruit Trinket Solar Power
Is size a concern? Are you planning on making a custom board?

If you just want to slap a few modules together, this is easy enough.

Along with the Trinket, get the USB/DC/Solar charger from Adafruit - http://www.adafruit.com/product/390 

The IC on the board will prevent any over-charging.

Sleep code is easy enough for the 85 - here is a more info: https://learn.sparkfun.com/tutorials/h2ohno/low-power-attiny