S.O.S. Holiday Travel

I need help. I'm taking a flight to spend Christmas week with my family in Puerto Rico. The flight is almost 5 hours long and we don't have a ticket for the baby. I shouldn't call her a baby. She's a 13 month old todler that weighs 25 pounds and measures 30.1 inches. Hence my fear of five hours with my adorable, friendly and very active Amelia jumping from her father's lap to mine, and screaming with delight at strangers.

We bought the tickets around May so we thought we could totally handle the lap ride. She was seven months then and had already traveled to PR twice, sleeping most of the flight. I know, rooky mistake on our part. The baby was bound to grow...a lot. It took a 2 hour flight to Florida at 9 months old to get us to buy a ticket for her on our next trip to the island in October. But December was booked already. It is what it is.

So I need suggestions. Amelia will be 15 months old in December and probably walking by then. She will watch a video on the DVD player but that's maybe 30 minutes. Plus she's too small to understand what the earphones are for, so we have to put the volume up for her. She's not used to sleeping on us anymore. She loves to just slide down from our laps to crawl all over the place. Confined spaces are not her thing. I know, you're happy you won't be sitting next to us.
My husband suggested we practice sitting on the couch for 4 and half hours with her and toys, dvd player and food. Funny, right? But we are desperate. Her pediatrician doesn't want us to go the Benadryl route which we would also have to practice since a lot of kids end up more excited than sleepy.

I'm hopping the plane is full of my fellow Puerto Rican families with little kids like mine and not evil people like the one traveler I encountered on my way back from Florida.
Your suggestions will be greatly appreciated. Ay Mama!