Look into hotel facilities & whether they have things for children to do (kids club)
Try not to travel for hours on end to get to your destination
Give yourself time for any delays on both sides.
carry toys & things to keep the child entertained at the airport/during the flight.
The child will be excited anyway & you may not need the toys!
Go somewhere where there's something for all of you to do regarding sight seeing/childrens interests, children get bored easily and may not enjoy what you want to do & so 'play up' - take this into consideration.
Would you go to a themed place i.e. Disney? There is a lot to see/do.
Always keep something for the child to eat/drink on you at all times.
Plenty of suncream & a hat. (for all of you)
Pack an emergency kit - plasters, wipes, insect cream/spray, sickness tablets, calpol, something in case anyone gets the 'runs' - like immodium, anything else you think you may need.
Stay in the frame of mind of an exciting adventure rather then working yourselves up and the child picking up on the stress vibes.
that's all I can think of for now.