Privacy

Protects one of your fundamental rights, or nearly all of them if you think about it, as well.

ham radio

Getting certified is super easy in many places and there are many hobbies within, such as electronics, building antennas, etc.

poetry

This hobby has to be the cheapest of all. Go to the park with a pen and paper, bonus points if you leave your phone at home, and let your mind wander :)