Buy a portable DVD player and some DVDs for her. I have 19 month old twin boys and we went last summer (when they were about 8 mos) on a family trip driving to NY to the Adirondacks for a family reunion etc. It was was worth the $100 I spent on the portable DVD player. I was able to use it in the car and also hook it up to a TV while we were there.
Also, Pack a few small board books, and small toys. I would consider making them toys that you won't care too much about (and neither will she) if you happen to leave them / lose them.
Depending on where you are staying, pack some safety items. Outlet plugs and the things to keep cupboard doors shut and closet doors shut are extremely helpful. I took these items when we went to FL this past Feb & it was good I did because the hotels we stayed at had nothing.
Plan to buy diapers and foods that she will want / eat when you get there for your stay. Only take what you will need for the driving portion of the trip.
If you are staying at a hotel, you can request a Crib (Pack N Play) for her to sleep in. I would also look into getting one of those portable Safety 1st inflatable booster seats - these are awesome. I was able to take 2 of them in a suitecase with us to FL. Wal mart sells them for about $20. You blow it up to inflate it for use and can deflate it so it packs flat. Great also for toting around to different places like restaurants, G&G's house, etc.
Take some of the Huggies washcloths that have the soap right on them and use those for her baths rather than lugging baby wash or buying it there. You can slip a few into a ziploc bag and toss it into your travel case.
Take plastic grocery bags and keep them in your car so you can use them for garbage and dirty diapers -- then toss them out as soon as you are at a gas station etc. in one of their garbage cans.
If you are going to be staying with relatives or near relatives, pack only the ESSENTIAL clothing you will all need. You can use their washer / dryer to wash clothes at least once while you are there so you don't have to pack as much clothing. This will really help conserve on space in your vehicle.
You can also consider asking your relatives to borrow a pack n play from a neighbor etc. or purchase one from a thrift store for you and reimburse them for it and then use it while you are there (if you are staying at their house) This will also help conserve space in your vehicle. A crib mattress on the floor will work too. You could do this for some toys too.
Bring your own sheets for the crib or PNP and at least 1 blanket and whatever she likes to sleep with. Most hotels do not have crib or PNP sheets, and I found that by bringing my own, my boys slept better because it smelled like home (the sheets from using our usual laundry detergent).
K.
Mom to Alex (10) Aaron & Aidan (19 month old twins)
Michigan