Vilros kits, incl. Raspberry Pi 3 Model B Rev 1.2

To install RetroPie, follow the instructions in the RetroPie Beginner’s Guide
Download + install the Raspberry Pi Imager:

Run the imager from your start menu, and use it to write the OS RetroPie 4.7.1 (RPI 2/3) to your MicroSD card, example here
Once it’s written, eject the MicroSD card from your computer and install it into the Raspberry Pi
Transfer some ROMs, according to the instructions here

Emulators

ClashMan Online GBA Emulator
Pizza Boy GBA Free for Android
Pizza Boy GBC Free for Android
Snes9x
VisualBoyAdvance

Other fun stuff:

Analogue is a company that is apparently planning to produce and sell updated versions of old gaming consoles. Everything is still pre-order only, but some of it looks really cool.
EZ-Flash Omega Manual and Setup Guide
Game Boy Advance games manuals
Gameboy Studio is “[a] quick and easy to use drag and drop retro game creator for your favourite handheld video game system.”
Goomba Color for converting DMG/GBC ROMs to GBA format
iFixit: The Free Repair Manual
Little Sound DJ music sequencer for DMG
Nintendo Game Boy Advance Champion Collection
RetroFlag GPi Case for Raspberry Pi Zero
r/Gameboy Wiki, because Gameboys are neat. You can make your own with Freeplaytech and Raspberry Pi.
r/SBCGaming, “Single Board Computer Gaming: Retro gaming emulation on single board computers, handheld portables, android devices and more…”
Stepper is a new 16-step sequencer for the Game Boy Advance (GBA).
Super Nintendo Entertainment System Champion Collection