I love this Board!
I've played verbal games with the kids for several years starting at about 18months. DS8 still loves to play but its evolved into 20 questions or the as he calls it, the 'guessing game.' (person, place or thing, animal, vegetable or mineral, etc). We played these games on short trips, long trips, waiting at restaurants, doctors offices, etc. The games encompass not only vocab but math or spelling too. They love to spell words - each at their own level (DD5, DS8).
One that might assist with vocab is what we call the synonym game. Name something and they try to come up with all the synonyms for it. Take the object - "shoes" and then all kinds of things can be said - sandals, boots, sneakers, galoshes, etc. Be flexible. Have fun and enjoy this special time with your kids.
A children's dictionary could prove useful. If pressed for a definition, you could model for her (as someone else posted) and look it up to obtain the definition. If you know of similar words, introduce her to those.
Finally, a great show on PBS is WordGirl which introduces some nice vocabulary. We searched high and low for a Wordgirl halloween costume this year, to no avail. Next time, if DD is still into it, we'll make one, I guess.
HTH and keep up the interactions with your DD.
Again, I love this Board!!! So nice to hear stories of other parents interacting in similar fashion. Not that there's anything wrong with it, but I just don't get how parents can "plug in" their child to DVDs/Videos on long trips instead of enjoyng the time together for at least a portion of the trip.