The concept of a solar powered flashlight seems very similar to the idea of putting a screen door on a submarine unless we add the qualifier that the flashlight is able to store energy. Then, the concept makes much more sense.
Solar powered garden lights are available at my local dollar store (the UK equivalent is something called Poundland if the internet is to be believed). These lights contain a solar panel, a nickel cadmium battery, an LED, and a tiny circuit board. It is easy to reconfigure the physical parts of this circuit into a durable and waterproof package, yielding a solar powered flashlight. Because the light turns on when the solar panel is dark, no switch is required, which improves waterproofing at the expense of making the device drain itself whenever it's not being exposed to light.
Step 1: Step 1: Disassembly
Disassembly of these devices is relatively easy; the solar cells are glued down with hot-melt adhesive, so I used a heat gun to melt it again. It's easier to take a look at the circuit board now. It contains an IC, part number YX80018 4W345, and a part that could be either a resistor or inductor (until I find out for sure by reading the application notes for that IC); if the part's a resistor, its value is 470 ohms +/- 10% based on the bands, but I'm willing to wager it's an inductor at this point, because that chip is converting 1.2V from the battery to ~3.6V (forward voltage of a white LED), and step-up voltage conversion typically requires inductors (ignoring charge pumps, piezoelectric transformers, Cockroft-Walton ladders, wimshurst machines, et cetera).
Step 2: Step 2: Reassembly
After dismembering the circuits, the components are rearranged on the pieces of clear plastic they are to be hot glued to (I found the plastic on the ground, no idea what type). The battery holder is to be replaced with hard-soldered connections to save space. I'm also replacing the wide angle LED with two narrow beam LEDs from my stock, to make the device a more effective flashlight. Experimentation revealed that the driver circuit could power 4 LEDs in series; I didn't try any more than that. I've arbitrarily decided to use 2 in this application. I've included a piece of twine embedded in the hot glue to make a lanyard.
"Yes, Virginia, you can solder directly to batteries" -- use this technique to make your own battery packs or avoid using battery holders, but don't come crying to me if you get covered in flaming lithium as a result. (Flaming lithium is a gorgeous shade of red. Hot, though.) Set iron temperature to 380 C, and scuff the ends of the battery with a file. *Quickly* apply the iron and (flux-core) solder to the freshly-scuffed surfaces. The adhesion should be practically instant; DON'T HOLD SOLDERING IRONS ON BATTERIES FOR MORE THAN 15 SECONDS. Once it's cooled you can test the security of your solder blob by trying to push it off the battery with your fingernail. If it's properly adhered you won't be able to. Then you can easily melt pre-tinned wires into the blob on the battery.