Soldering irons are good! Your compiler is your friend! Do you wonder why snakes are named after programming languages? Then this is your stop.
Read ETOPO1 bathymetry. Elevation or bathymetry databases are the kind of things you don't want to find yourself without. The ETOPO1 database is a relatively recent global data set; download a convenient Matlab function here to extract data for your favourite region! Also works in Octave.
QL Keys (1). The Sinclair QL brought to modernity. Follow this link to find one way of giving this totally cool eighties computer - or rather its exterior - a new and meaningful place in your life!
QL Keys (2). The follow-up. After building a keyboard matrix-to-USB converter there is some room to spare in the QL case. The solution? Put more stuff inside!
Atari joystick to USB. Attach a classic Atari-style joystick to your modern-day computer to make your emulated retro-gaming experience even more realistic. (Note: totally obsolete; you can get better solutions nowadays.)
Using the MOS 6581 SID chip. The MOS SID (Sound Interface Device) is one of the reasons for the success of the Commodore 64. More advanced than the sound chips found in most other 8-bit home computers of the era, it still finds use and fans today.
MOS 6581 follow-up. Adding features to the FrankenSID project. A box, LED's, MIDI. Make it more or less usable!
Revitalize the Silicon Graphics Indy "Dallas" timekeeper. Your Silicon Graphics Indy workstation may suffer from a forgetful PROM chip (mine did). With some work, it can be restored to working condition.
Use a Commodore 64 as a Bluetooth keyboard for iOS devices. With a few extra parts and some software a C64 can be turned into a Bluetooth HID. No Commodores were permanently injured for this modification.