A cell phone switching charger would be great in this application.
I like that, and the USB.
Newer cell phone switching chargers are about 3.8V, but mine is 5V. USB is about 5V.
Newer white LEDs tend to operate near 20ma with 3.2V.
Meaning not enough voltage for series (easier for this application).
I would suggest calculating the LED at 16ma and 3.2V when choosing the resistor. Then actually measure the ma.
(5.0V - 3.2V = 1.8V in the resistor.
1.8V / 0.016A = 112.5 ohms, 110 or 120 ohm is a good place to start)
If going through this much surgery, I would also suggest changing the factory LEDs to something brighter (if they are 5mm and Not surface mount). Solar yard lights tend to have lame LEDs in them.
More light for the same power.
G-