Disposable changing mat for DD to sit on on the plane seat. When she threw up one hour into a nine hour flight it was great to know it had all been caught and she had a clean seat to sit in for the rest of the journey. Also the seat didn't smell for the rest of the flight.
A bag full of attractive snacks and small, cheap activities for the toddler on the plane. I cannot contemplate a mere 2.5 hour trip to Portugal with my wiggly 2.5 yr old without a bag full of fruit yoyos, mini cheddars, juice, pretzels, breadsticks chocolate and apples.
Do you like using slings? You can buy or borrow/rent a lightweight wrap sling (Calin Bleu etc) that might be a shade cooler than a carrier like an ergo. That said, I'm about to take my toddler Connecta somewhere hot and I'm choosing to forget how sweaty it can get!