An idea I’ve been wanting to do for a while is to use an old phone as an IoT device. A second-hand smartphone is potentially a cheap way to get a processor, screen, and storage. The phone needs to be hackable – I think an operating system like LineageOS or JanOS needs to be installed on it. LineageOS has the best support for different phones. In order to be useful for IoT, the base phone should be extensible to add sensors or micro-controllers.
Jay Carlson has a superb overview and comparison of various cheap microcontrollers. All the components he was looking at were less than $1. He evaluates 21 different chips and compares specs, performance and development tooling. It’s well worth checking out if you are looking to add a cheap micro controller to a project.