The messages have to broken in order, as each one helps clue the key to decrypting the next.
The reference to salad dressing indicates that this is encoded with a Caesar cipher or rotation cipher. Running through the values, we find that rotating each letter back seven places in the alphabet gives this:
A MAN AND A WOMAN STANDING THE MAN IS HOLDING A FARM EQUIPMENT NICE HOUSE IN THE BACKGROUND
TWO, given ONE
The deciphered text from ONE is a reference to the Grant Wood painting American Gothic. Decrypting this text as a Vigenere cipher with key "AMERICAN GOTHIC" gives:
ANICEEVENINGINAPARISIANISLANDBEFOREMONDAY, that is
A NICE EVENING IN A PARISIAN ISLAND BEFORE MONDAY
THREE, given TWO
The deciphered text from TWO is a reference to the Georges Seurat painting A Sunday Afternoon on the Island of La Grande Jatte. Decrypting this text as a Vigenere cipher with key "ASUNDAYAFTERNOONONTHEISLANDOFLAGRANDEJATTE" gives:
CHOO CHOO I COME OUT A FIREPLACE AND STAB PEOPLE
FOUR, given THREE
The deciphered text from THREE is a reference to the René Magritte painting Time Transfixed. Decrypting this text as a Vigenere cipher with key "TIMETRANSFIXED" gives:
I WANT TO SEE THEM ALL
FIVE, given FOUR
The deciphered text from FOUR is a reference to the fact that all four mentioned works of art are part of the permanent collection of the Art Institute of Chicago. Decrypting the final text as a Vigenere cipher with key "ARTINSTITUTEOFCHICAGO" gives:
ONEPMTHISSUNDAYBYTHERIGHTLIONDONTBELATE :), that is
ONE PM THIS SUNDAY BY THE RIGHT LION DONT BE LATE :)
As you can see on the linked Wikipedia page, the Institute's main entrance is flanked by two bronze lion statues. With your meeting time, meeting place, and date planned out, you should have lots of time between now and then to prepare.
If you want to make a good impression, I'd recommend reading up a little on some more famous pieces you might see there; there's quite a lot.