I would avoid the Benadryl; it will as other say here make her either sleepy or hyper, and do you really want her in either state when she arrives at a party, grandparents' house, etc.? Dramamine can work on adults but I have no idea how it works in kids that young and it may have side effects; I'd definitely ask the doctor first about that one. You can't necessarily drug her for every trip so explore all the options.
Try the old-fashioned stuff first if possible before medications:
--Keep the windows cracked for fresh air, yes, even in winter, and do not have the heat on too high. Being hot and airless encourages nausea. Have her take off coat, sweater, etc. before any ride so she's comfortable and cool.
--Get her a LIGHT snack like crackers before she goes but not too close to drive time. Have crackers on hand. Nothing too salty or heavy. Have water available.
--Ensure she is not looking down at books, electronic toys etc. She needs to look up and out the windows as much as possible! It's the same idea as on a boat -- look down and you get seasick, look up and at the horizon and it's better for your brain and stomach. Try playing games with her, especially "I Spy" and licence plate spotting games that make her look outside the car (these will also distract her from thinking about "Oh, no, I'm in a car, I always get sick in a car" -- with some kids, if they think they will be sick, they WILL be sick).
--Do try the wrist bands etc. that target pressure points. These work well for some adults I know and not at all for others but are non-medicated and worth a try.
--Stop more frequently so she can get out, walk around, etc. It is not as convenient, naturally, but may help. Good luck and try to do without the drugs if you can.