It's crazy how similar photography is to playing guitar...it's artistic and creative at its core, highly technical in its application, what's "best" for any given purpose is highly subjective and very personal, it's hard to avoid "G.A.S." once you get started and it takes a lot of time and practice to truly master it!
That 105mm is great! I have a couple nice prime lenses in the 35-105 range for use in more controlled environments, special situations where I need the absolute best image quality I can get or when I just feel like messing around with a fixed focal length for fun. It's hard to beat the versatility of the zooms I mentioned (they are top-quality glass, not "all-in-one" consumer type zooms), but I rarely leave home without at least one prime in the bag.
Speaking of bags, when I'm carrying a minimal kit (Camera + 1-2 lenses and some simple accessories) and need something that doesn't scream "photography equipment inside", I have grown to absolutely LOVE the
original Lowepro "Passport" Sling! I used it in an urban setting for a solid month before spending 2 grueling weeks traveling across Asia in November and was quite impressed. My wife liked mine so much that we got her one before we left. She took out the camera insert (we only took one DSLR overseas) and she used her bag as a regular satchel the whole time. Love the fact that it expands when you need it to and collapses back to a smaller size when you don't need the extra room. If the OP is looking for a nice basic bag to give the girlfriend for her new camera, I can't recommend the Lowepro Passport enough: