Lights are easy. Computers can be finicky, but bike shops know how to install them, so let them. You may as well learn how to install the saddle, because there's a fair chance you'll want to adjust it later (tilt and setback), and it's easier to see how to do that when the seatpost is off the bike and upside down.

Fenders, do it yourself. Yes, they're finicky to install. But if you have any mechanical abilities at all, whatsoever, to the point of being able to put a cap back on a pen, you can figure out how to put them on just as fast as most LBS employees. To be fair, unless they've been around for quite a few years, they've never had to put them on.