Boredom is the biggest challenge with a toddler and not understanding the rules for staying seated and buckled (in my experience). For a 3 year old, I would pack: snacks, change of clothes (or diapers/wipes depending on the potty training situation), toys (including a new toy or 2 for novelty), tylenol (I always give proactively in case of ear discomfort), benadryl and/or dramamine (in case of an allergic reaction and/or motion sickness) and any prescriptions the kids take. You might also want to be sure you have a list of medications, doctors and phone numbers and other important information. I put a sticker on my son with his name and my cell phone number just in case he ran off/got lost when I traveled alone with my kids. Plus, I am a little neurotic, so I also always have disinfecting wipes and hand sanitizer. I usually fly JetBlue, so I also pack toddler headphones. My son loved to keep his toys, snacks and books in his own backpack.