I also have a canon 6d and have the 24-105mmL4usm lens. It is a good walk around lens.
Indeed. Almost everybody with a 5D or 6D has the 24-105. It's the obvious walker for a full-frame Canon. I bought mine as the kit lens on my second 7D, on which it functioned as a good moderate-range telephoto, But the real reason I wanted it was that I'd already have it when I eventually went FF.
If you bought the camera new you kind of missed the boat not getting the 24-105 as an included kit lens. Do not pay full price at this point. As stated above, go for one of the "white box" deals, where someone did buy the lens as a kit lens with a camera, and is splitting up the kit. You can still get the lens for around $700 that way.
What you use your camera for is likely to be much different than me, but I’m happy to share with you my preferences.’
My partner and I use two lenses regularly, the EF 24-105mm f/4L IS USM for everything from landscape to bird photos, and the EF 70-200mm f/2.8L USM for portraits to architecture shots. These are generally our “walk-around” lenses we go out with, though she loves flowers and uses her EF 100mm f/2.8L Macro IS USM as a third lens she usually carries with her when we go on walk-about. We both carry the Extender EF 1.4x III and Extender EF 2x III.
I hope this helps.