Must have -- disposable changing pads. I bought the Huggies ones online and they were GREAT to have at the parks!
Must have -- quart size and gallon size storage bags. I bought the Target brand ones with the zippable (is that a word?

) top. Great for packing outfits, for packing snacks, for storing leftover food for snacking later, for disposal of smelly (!) diapers, etc.
Must have -- disposable sippy cups. Who cares if they get lost (which they will)?
Must have -- sunscreen and diaper cream. We brought Triple Paste in case of a bad diaper rash.
Must have -- a small roll of duct tape. We used it to tape down electrical cords in the room, to tape down a table cloth at the California Grill (under the table), to tape rain ponchos down to DD so it didnt blow in her face, etc. You can even tape over electrical outlets with those saftey plugs if you have a VERY inquisitive one. I now have a small roll of duct tape in my diaper bag at all times.
In the Why Did I Pack So Much category -- toys. It was silly to bring so much stuff with us. I would REALLY cut back on this stuff as you end up needing so little -- plus you are going to buy stuff there anyway.
In the Wow, What a Great Tip category -- one of those over the door holders -- I bought ours for $1 at the local dollar store. WHAT A GREAT IDEA! We stored bathroom/toiletry things in there. It helped us get stuff off the vanity so when DS was cruising the room (he didn't walk until 13 months but was definitely cruising at 10 months) he didn't pull stuff off the vanity.
Good luck and have fun!
Karen