Blinking LED at two different frequencies
Firstly I would advise using the CMOS version which operates at a lower voltage than the BJT version 555.
The key to getting different flash rates is using the control pin. This is connected to the internal resistor ladder (3 x 5k or 5 5 5).

Adding a jelly bean NPN and two resistors you can easily switch between the two rates. When the transistor is turned ON the flash rate goes up. With the transistor OFF it goes down. Now all that is required to complete your project is some form of comparator switch to operate the transistor e.g. a low voltage rail-to-rail op amp.