We did it a few times when our kids were that age--really not bad, I promise
Be sure to arrive at the airport early enough nto to have to rush with your little one to the gate. Once you are all checked in and have found the gate, don't SIT and hang out at the gate. Stay up and active a the airport. If there is an empty gate near your own hang out there. Play movement games right up until boarding to get the wiggles out while you can (things like Ring around the Rosie, Simon says, head shoulders knees and toes, etc.).
As others have said, once on board hand out little things to do and eat one at a time--and clean up the old before each new. My kids never got to play with my office supplies at home so I would bring them on flights and they LOVED them. Things like post it notes (in different sizes and colors) and masking tape can entertain a toddler for quite a while

Other hits with my kids were pipe cleaners, slinky, model magic (play dough is too messy for a flight), favourite story books, fingerpuppets and magnadoodles. We also took coloring items and colored and even made the flight sickness bags into puppets a few times
If the seat belt sign is not illuminated try to get up and take a walk up and down the aisle about every half hour (we took a walk between each new toy or treat when allowed).
IF your child can have lollipops without choking they are good for take of and landing to ease ear pressure. If not, a sippy cup or bottle will help.
Do NOT use Benadryl or any other medication you have not given your child before because you do not know how s/he will react.
Alos, pack extra wipes for hands and face and 2-3 changes of clothing. Kids can be more irratable when they are sticky and dirty. Pack jammies to change into for a nighttime flight.