We took dd to Majorca when she was 9mths, we took formula milk and bought 5L bottles of water which we boiled. I also took some washing up liquid for washing bottles etc. I used boiled water for this as well, since she was crawling and she was into everything I didn't worry too much about sterilising.
As for food, we were half board and she ate the same food as us without any problems. If we went out for the day we popped into the local supermarket and bought a jar or two of heinz baby food.
Definitely take a harness for the high chair, we found that after a couple of days the waiters would keep a high chair for us as they were in short supply.
We also took dd to the Costa Dorada when she was 22mths, had a great time. You shouldn't have problems finding anything if you run short. MIL left the sunshade for the buggy on the bus, we got our rep to phone the bus station but no one could find it, so we went to Tarragona, found a Mothercare type shop and bought a new one.
Agree with not taking too many toys, take as many as you need to keep dd amused on the plane and buy more in resort.
Last thing, take plenty of wet wipes!