We had a back pack that also was our stoller when our DD was 8 1/2 months old. It worked great (it was the only stoller we had the whole trip and we did fine.) It was made by InStep and cost 50$ at Target. She sat very upright in it, but was able to fall alseep and it also has a basket with it. It comes with a funky canapy for shade, but you don't have to use it. It also was very light wieght, folds up small and easy on the bus. I was a tad nervous not having the "normal' stroller with us, but it worked great.
She (and us) loved it. We were able to converet it into a back pack carrier with her still in it (had to be careful not to get her feet caught in it) and put it on my hubbys back. My husband was always the one to back pack her. I used a on the hip carrier, she never liked baby bjorn for some reason!
I recomend using it a head of time and getting both of you used to it.
DD was only 14 or 15 pounds at the time. We used it up until she was 18 months old, then she was a bit too big for it, her feet could touch the whells and she would drag them. It also was a great help last summer at the beach, baseball games and other places it was difficult to bring a stroller. Really it gives you the best of both worlds.
We used it as the stoller most of the time, but it was handy when we needed it. It did the job of 2 things, but only took up the room of one. Also she loved being up high and being able to see things better.
Good luck! (if you want I can post a picture of her in it.)