The dark portal puts you into the Palace area. After a quick scene, you gain control of Roxas. There are lots of Scarlet Tangos and Loudmouths around, so be prepared to fight. There also a few chests and an Ordeal Badge to snag as well. When you're done fighting here, proceed to Agrabah Street. There are more Heartless here, as well as a few more chests. On the highest roof in the southwest corner sits a treasure chest and an Ordeal Blazon, but you won't be able to reach them without High Jump. If this is your first time through, you'll have to come back for them later. Proceed to Agrabah Gates and repeat: destroy the Loudmouths and grab the chests. Also, keep an eye out for the Bulky Vendor in the western ally, near the closed-off entrance to the Cave of Wonders. When you are ready, head out the main gate exit for the Sandswept Ruins.
The new Sandswept Ruins is a very large area. You're going to have to watch carefully for chests and the Unity Badge, as they can be very easy to miss in the vastness. Also, the giant Antlion trying to destroy you does not help either. The battle is a little time consuming, but it will eventually go down. Afterward, return to Agrabah. There will be one more scene at the Palace, and then you can RTC.