We just got back (a week ago) from our 7-night trip with DS (14 months) and DD (she'll be 3 on 10/23). I can't choose just one thing - sorry - but here are the things we found were essential:
1. RayShade for our double stroller - kept the kids completely shaded from the sun. My kids always had a shaded respite from the intense sun thanks to our RayShade, and never got burned.
2. Anti-bac hand wipes (for hands, table tops, highchairs, etc.)
3. We bought one of the Disney mister fans (a deal at $17...HA!

) on day 1 because it was SO hot - and found that to be an essential...not only for battling heat, but it also gave both kids something to keep them entertained/occupied.
4.
CROCS!!! They were the best for DD! They were comfy, easy to walk/run in, quick to dry, easy on/off, etc. (oh, and she LOVED wearing them) Really. They were THAT great!
5. For identification I had 2 silicone bracelets made like the "LIVESTRONG" bracelets. I got them on ebay, but you can also buy them directly
www.rekeydesigns.com. I had the following engraved on the bracelets "Mommy & Daddy (###) ###-####" with our cell number. Got one in pink and one in red to match DD's park outfits

and she wore one each time we left the hotel. She loved them, they looked adorable, and we all had the peace of mind that she was "branded" should we get separated. We coached her before leaving on how to use the bracelet if we were separated.
Sorry for the book, but these all were our essentials.