Their first stop is the city of Ise, located on the Shima Peninsula in Mie Prefecture. The city is home to the Ise Shrines, Japan's most sacred Shinto shrines. The choice of Ise was Nekoi's recommendation.
The main character of Gate 7, Hana, comes from Kyoto and she is the daughter of a temple caretaker. CLAMP will most likely take pictures and collect other kinds of data to use as drawing references.
Below is Satsuki Igarashi's original entry:
Translation here. She's basically saying that since Tsubasa is now over, they now have more free time and thus they are out on a trip to start collecting data for their next work (I am assuming that is Gate 7).
So happy to see that things are finally moving forward with Gate 7!