You can purchase blinking LEDs. Connect one of these up to a solar panel and that will work with enough light (and a suitable panel).
For safety you might need to place dome diodes (or a zener) across the LED to protect it from higher voltages. The flashing LEDs typically do not require a series resistor.
If you want it to flash even in the absence of a lot of light then you should google "BEAM robotics flashing LED" to get some ideas.