Hi there, your trip sounds really exciting!
I've been flying with my kid (now 3.5) since she was an infant, but I think many of these still hold true:
It's a matter of personal preference, but I don't bring DD's car seat along. We used to use the CARES harness but she is at the height limit now at 42", so I expect your DD would be as well. I try to stick to the truth as closely as possible, so now that she's graduating from the CARES harness, she gets to sit like a "big kid" and the grownups. And then she just sits in her seat on the DME and shuttle buses around the resort.
General tips for riding on the plane, I guess I'd suggest you prep her by talking about it, about what the plane is going to do, and that the pilot/captain and the crew will be in uniforms ("like police officers") and they are they to ensure our safety. They, not mom and dad, make the rules on the plane, and it is very important that we listen to and follow all the rules. There are going to be certain times that we MUST sit with our seat belts buckled and it is to keep us safe, etc. Even Mom and Dad have to follow all the rules, etc etc (just like riding in the car, we need to be buckled in all the time, etc).
I wouldn't medicate unless your daughter suffers from motion sickness - Dramamine can sometimes make kids hyper and mess with the sleep schedule.
We did a combination of games etc on the tablet, sticker books, lots of small snacks, etc. some flights you may have personal entertainment screens; those are always good for DD to just press buttons and fiddle with. What works for us - DD gets limited screen time on the ipad as it is, and never gets to watch full-length movies except on the airplane. So now, she just watches Frozen over and over and over ... it's really easy. Try giving her as much of a preview of the plane flight as you can, make it into an exciting part of the trip as well (and not just something you have to endure), and if you have a window seat, she may be cool looking out the window for a good part of the trip too. If all else fails, the flight attendants are usually really good if you take a walk to the bathroom and let her ask some questions about the flight, or the plane.
Oh- if she has a comfort item - a blankie, a stuffed toy, BRING IT. Even if she doesn't use it much at home, she may like having something familiar to snuggle up to.