BENADRYL! - NO! I'm TOTALLY kidding. I've heard about that on TV or other media things, but I always thought it was pretty mean. Anyway. We flew from Phoenix to NYC when my first daughter was about 12-13 months, and from Phoenix to Salt Lake City when she was 3 and with a 1 1/2 month old daughter. We also drove from Phoenix to Vegas with a 2 year old daughter and twin 4 1/2 month old boys. All trips went extremely well. When we flew to NYC, she sat on our laps (mine or my husband's), could look out the window, eat her snacks we brought and drank the milk we brought. She was addicted to her pacifier at the time, so we used that to help her ears with the elevation. At the time, she was sleeping in a crib at home, so we packed her playpen and set that up in the room. At naptime and bedtime, we just put out the lights and made the room quiet. Of course, she never liked going to bed, so this time was no different, but once she fell asleep, she slept well for the time she was supposed to.
The time we flew to Salt Lake, the now 3 year old was allowed to bring her own little bag (carry on) with some of her favorite toys or activities, and a sippy cup (all airport security approved - no unapproved liquids, sharp objects, etc.) She got to pick everything to go in the bag and pack it herself. She was very proud to be responsible for it. It helped keep her entertained on the plane as well as in the airport. We also packed the diaper bag with some of her favorite snacks. She didn't seem to have any trouble with her ears that time. The 1 1/2 month old just slept pretty much the whole time. As we were taking off and inclining, we fed her a small bottle to help with her ears. She didn't seem to have any problems. As for sleeping, at that age, the 3 year old was already sleeping in a regular bed at home and thought it was the coolest thing to be able to sleep in a great big queen bed in the hotel room. The littlest just slept in her carseat. She did fine.
When we drove to Vegas with my 2 year old daughter and twin 4 1/2 month old boys, the hotel sleeping arrangements went fine. On the first night, we actually stayed in a very small, one bed hotel room, and the 2 year old slept in the bed with mom as dad slept on the floor. The twins slept in their carseats. The rest of the time, she had her own bed and loved it (she was in a twin bed minus the frame since 18 months, so the bed was no problem for her.) Your little one may enjoy the time with you if you let her sleep in the same bed with you. Or you could pack a playpen. Whatever you feel she might be comfortable with. You may try packing one of her favorite blankets or other comfort item (assuming it's small) to help her feel more relaxed in a strange place. Good luck with your trip.