555 timer as a humidity sensor
You could use a counter after the 555. For example, a 74HC4040 would allow you to reduce the frequency by as much as 4096:1, so you could use a reasonable resistor value in the 100K range (assuming a CMOS 555). If you are using higher than 5V, a CD4040 would work. 



You can pick whichever output you like to select the output frequency in steps of 2:1. For example, if the input frequency is 20kHz, you could select 10kHz, 5kHz, 2.5kHz, 1.25kHz, 625Hz, 312.5Hz, 156.25Hz, 78.125Hz, 39.0625Hz, 19.531Hz, 9.765625Hz, or 4.8828125Hz by selecting one of outputs QA to QL.