Hi! We are due with our 2nd baby in June! :) Congrats on your new addition!
Our little girl is 3, and we were dealing with the same thing - I wanted something that she **could** ride in (if we're somewhere like the zoo), but that wasn't a waste of space if she wasn't in it. I could just see either getting a huge double stroller and her never riding in it... or having just the infant travel system for the new baby, and Emma getting tired and needing to be held or want to ride, and we have nowhere for her to sit...
SO... we got the Joovy Caboose. (http://www.amazon.com/Joovy-Caboose-Tandem-Stroller-Orang...)
The space in the front can accomodate a child who wants to sit, or an infant carrier (with the attachment). Then, in the back, Emma can either stand on the little ledge (with a strap that goes behind her back to hold her in), or sit on the seat that would face mommy or daddy pushing the stroller. I am SO excited about using it. She's excited because I told her by sitting there, she can help us with the baby. :)
Also, yesterday, we went to the Dallas Arboretum with some friends who have a 2 month old baby girl, and they left their stroller at home, so we got to try out our new one! It was PERFECT! The baby carrier/seat fit great in the front, and Emma was able to ride when she wanted, and then get out and walk... and our "double" stroller was not near as bulky as our other friends who had their Graco travel system with them (which is what we had with Emma - either the Quattro Tour or the Metro-Lite). It was easy to operate, too... I love it! :)
Have fun shopping and good luck with the arrival of your new little one! :)
S.