Keep a special box of car toys that you'll use only in the car. If these are interesting enough, they may hold her attention.
Tape or hang toys for viewing. You can do this on the back of the seat that your baby is facing or string an array of lightweight toys from the ceiling using heavy tape and yarn. Place them just at arm's reach so that your baby can bat at them from her seat.
Make a car mobile. Link a long row of plastic baby chains from one side of the backseat to the other. Clip new toys onto the chain for each trip.
Hang a made-for-baby poster on the back of the seat that faces your baby. These are usually black, white, red and bold primary colors; some even have pockets so you can change the pictures. (Remember to do this, since changing the scenery is very helpful.)
Experiment with different types of music in the car. Some babies enjoy lullabies or music tapes made especially for babies and young children; others surprise you by calming down as soon as you play one of your favorites. Some babies enjoy hearing Mom or Dad sing, more than anything else! (For some reason, a rousing chorus of "Rudolf the Red-Nosed Reindeer" has always been a good choice for us, even out of season!)
Try "white noise" in the car. You can purchase CDs of soothing nature sounds or you can make a recording of your vacuum cleaner!
Practice with short, pleasant trips when your baby is in a good mood. It helps if someone can sit near her and keep her entertained. A few good experiences may help set a new pattern.
Try a pacifier or teething toy. When your baby has something to suck or chew on he may be happier.
Hang a mirror. That way your baby can see you (and you can see your baby) while you are driving. Baby stores offer specialty mirrors made especially for this purpose. When in her seat, she may think that you're not there, and just seeing your face will help her feel better.
Put up a sunshade in the window. This can be helpful if you suspect that sunshine in your baby's face may be a problem.
Try to consolidate trips. Trip-chaining is effective, especially if you avoid being in the car for long periods of time, and you don't have many ins-and-outs.
Make sure your baby hasn't outgrown her carseat. If her legs are confined, or her belts are too tight, she my find her seat to be uncomfortable.
Try opening a window. Fresh air and a nice breeze can be soothing.
If all else fails . . . take the bus!