One of the things I try to do is monitor the National Simplex Calling Frequency for 2 meter FM: 146.52 MHz, especially when mobile. It is fun to see who might show up on 2m simplex: SOTA operators, hikers, mobile stations, campers, etc. See these two posts on operating ideas: A Simple Wilderness Protocol: 146.52 MHz and The Use of 146.52 MHz.
It is also fun to say hello to another ham when you pass them on the highway. While you may see their call letter license plates or notice their mobile antenna, you may not know what frequency they are monitoring. The Noise Blankers Radio Club has solved this problem — just put this sticker on your vehicle.
After poking around cafepress.com, I found some additional options for indicating your radio frequency:
The APRS frequency:
And there’s this magnet for your refrigerator:
If you don’t like any of these, cafepress.com makes it easy to create your own. Go for it!
73, Bob K0NR