I just recently finished up blogging about my experiences working through the experiments in the Make: Electronics book by Charles Platt. It took about a year and was 100% fun... I highly recommend this book to anyone wanting to really learn about electronics and circuit building... I learned more from this single book than I've learned anywhere else.
Now that the Make: Electronics blog is finished, I've decided to continue forward by digging into the Arduino. If you're not familiar with it, I don't think I could do an explanation justice, so I'll simply point you to the official Arduino site as well as to the hundreds of Arduino projects that exist across the Internet - Instructables has some here, Make magazine has some here... do a Google search and you'll find more information than you'll ever have time to read.
I selected the "Beginning Arduino" book by Michael McRoberts - it comes highly recommended and after reading through the first 50 pages or so, I knew it was the book for me. It has 50 projects that start from the extremely simple "Light an LED" all the way up to building full-fledged robots with servos and other sensors.
You can follow my work as I tackle these 50 projects over at HandsOnArduino - I post my notes, pictures, and videos... and I'm keeping a spreadsheet on the blog that tracks my purchases for the projects.
I must admit that my experiences with the NXT over the past 5 years have really caused me to want to go deeper into electronics and robot building, and I'm sure there are many kids and adults who feel the same... I'm hoping that documenting my experiences with these books will help others to see what's involved and maybe encourage them to continue forward...