Raspberry Pi/Arduino Summer Project

Hello all! I'm not really new here, but I'm trying to become more active.

I'm looking for a fun summer electronics project!

Since I got home from school for the summer I've been pretty bored, so I started looking up an old interest--electronics! I had one of those electronics kits for kids when I was younger. Now I pretty much know nothing on the hardware side (I know C++ and am currently learning Python), but I know that I'm going to buy a Raspberry Pi 2 and an Arduino starter kit and make something! Any suggestions for good, moderately easy projects?

I'm a long-time electronics hobbyist who has somehow managed to avoid getting an Arduino--though I've done plenty with other MCUs and embedded platforms. If you're just getting started with electronics, though, I might recommend a good book to go along with your Arduino: Practical Electronics for Inventors, by Paul Scherz. It assumes very little, makes liberal use of easy-to-understand analogies, and offers just a ton of immediately practical advice and examples. You'll like it, I promise. It's my favorite electronics text of all time.

It really helps if you pick something you actually want/would use. We can suggest all kinds of random make work projects, but it's way more fun if you are working towards a larger thing that you actually have a use for, even if you are only working incrementally/doing a small part of it.