I am biased.....but get you a Corvette.
Anything from the C5, C6, C7 category. Very easy to work on from engine, to suspension, to interior, etc.
You can also get into tuning with software which is very fun to do. You can make it as mild or wild as you want it.
I know where you are coming from as I am kind of the same way. I
have to have a project or something I am working on or towards. My wife thinks I'm OCD, but I love a good project car and a sense of accomplishment.
That urge was satisfied in the past by restoring old cars...and that could be another option for you. An old Mustang with a drive-train swap or something along those lines.