Undella Town is where you will find Cynthia from Sinnoh, and you can get HM06 Dive and a number of other items here.
Cynthia from Sinnoh lives here. She has level 75 and level 77 pokémon, so don't fight her unless you are prepared for that. You don't get any items for defeating her or anything, but you do get money and experience.
The woman looking out to the ocean near the Pokémon Center will give you HM06 Dive. You can use this in Undella Bay to the east to find underwater treasure.
Talk to a person in the Pokémon Center to receive the Prism Scale, which you can give to Feebas to evolve it into Milotic when traded.
Fight and defeat the School Kid outside of the mansion. He'll let you into the mansion, where his father will buy rare artifacts from you that you find using Dive. You can also fight members of the Riches family. The fights get more difficulty as you go on, but they also give you more money each time.
Check the bushes outside of the mansion for a Big Nugget. You can sell it for a lot of money.
In February, June, and October, there will be a guy in one of the houses near the mansion who will trade you a Munchlax if you give him a Cinccino.