I recently did attempt to do some reverse-engineering. Didn't finish the project though,

I own one of those Tomtom sport watch, which tracks different metrics, and export it all to "the cloud" using a proprietary tool.
At some point, that tool stopped working on Linux and I couldn't see my cool graphs showing all the pain I have to do sports.

I used mitmproxy to catch all the (encrypted!) traffic between the software and the cloud, and try to figure out what was happening under the hood. That was a great experience, and I managed to upload some tracks extracted from my watch to the cloud. Extraction was done using an opensource CLI tool, so all was great.

I gave up on it when I realized that the software has a way to generate some kind of "token" that must be registered onto the official website before synchronizing data. At this point I managed to fix the software so it runs again on Linux, so I just use that because it's easier and gets the job done in a more easy way.

Was pretty fun though !