I'd focus on Mossdeep, Sootopolis, Slateport and Pacifidlog City.
Mossdeep has the space center which is amazing, it's also a relaxing spot.
Sootopolis has the Cave of Origin too, as well as it's Gym Leader Juan.
Pacifidlog is very amazing, that's all I can say about it.
Slateport is perfect, it's the biggest city in hoenn(I think), it has the harbor, and it has lots of markets.