Digging through your bag for keys can be annoying, especially when you're biking and need to lock up frequently. A carabiner solves that issue. First, add keys to carabiner. Second, attach keys to pant loop (or outside latch of bag). Third, life becomes easier.
While this Bison carabiner ($5) is NOT approved for rock climbing, it works great as a keychain and doesn't cost a lot.