Her fancy was running riot along those days ahead of her. Spring days, and summer days, and all sorts of days that would be her own. (19)
The "days ahead" that will belong just to Mrs. Mallard seem almost too many to count. It's as if she can't even hold in all the glory and the goodness of those days, stretching out, days where she can do whatever she wants.
Some one was opening the front door with a latchkey. (21)
Even though people are inside the house, it's still securely locked. In order to enter, one has to be admitted or possess a key. So, even in a time of extreme emotional feeling or loss, propriety is observed, things remain orderly, and the doors stay locked. In addition, we can tell immediately that whoever's entering in this sentence is someone who belongs or has the authority to be there, since he (in this case) has a key.