A display constructed out of common materials, giving you HMD overview of a connected sensor (through bluetooth).
Idea can probably be converted slightly for a more thorough / programmable HUD on the glasses.

Prototype presented here was roughly 70€

A free and cloudless replacement for your gadget vendors' closed source Android applications. See the list for supported devices. https://gadgetbridge.org

search document dumps: ingest and explore in one extensible framework - GitHub - newsdev/stevedore: search document dumps: ingest and explore in one extensible framework

Creating a weather station for air pressure, temperature, rain, humidity (soil and air).

List of quantified self resources - some outdated and some missing (Grafana would fit into many qs deployments, datasette, etc) but not bad for an overview.

An extensive introduction to all things Arduino and circuitry - goes through setting up and using Arduino, various electrical components, foundations of electrical engineering, analog parts, communication with computers, and more.

karlicoss of the data liberation project HPI explains how to best store and access data moved from various points in the cloud/web/internet to your drives and why databases might not always be the best choice.

TLDR.
Save your grabbed data without any manipulation.
Let the manipulation happen every time you access/interpret the data.
If you have slices of data (mostly time frames), don't try to merge them on disk but save as extra files and merge on access/interpretation as well.
You can make use of databases for access caching since the last points generate some overhead for each access.