As soon as you touch ground, Hau will challenge you to a battle. There's no declining, but fortunately he'll heal up your party.
Pokemon Trainer Hau
|Flareon (if you picked Rowlet)||28||Fire|
|Vaporeon (if you picked Litten)||28||Water|
|Jolteon (if you picked Popplio)||28||Electric|
|Brionne (if you picked Rowlet)||29||Water|
|Dartrix (if you picked Litten)||29||Grass/Flying|
|Torracat (if you picked Popplio)||29||Fire|
Hau's Pikachu has evolved! The Alolan Raichu is a formidable foe, so exercise caution. Now that it's a Psychic-type, it's also weak to Dark, Bug and Ghost moves, in addition to Ground.
Raichu isn't the only change; your cheery rival has smartened and grabbed an Eevee evolution that's strong against your starter Pokemon. However by now, you should be smart enough to deal with it.
Finally, to handle his starter Pokemon, send out your own starter Pokemon. Easy peasy. However, his starter does have a matching Z-Crystal, so bear that in mind if using a different Pokemon.
Afterwards, your buddy Rotom will let you know that another island Pokedex has been added. From here, you're free to wander around, within reasonable distance of Malie Garden that is. Heading east from here will take you to Ula'ula Island's ferry terminal.
Inside, you can book a ferry back to Heahea City or Aether Paradise. Although you can simply Charizard Glide back to Heahea City and there's nothing to see in Aether Paradise. In the north-east corner is a vending machine selling Soda Pop, three times. Redundant, much?
West from here is the Pokemon Centre of Malie City. Inside, the right Poke Mart clerk is selling a bunch of decent TMs for 10, 000 Poke Dollars each. By the cafe area is a kid who got a Mewtwo from Wonder Trade. Sorry, kid, but it's probably hacked...
Outside, continue west along the main road, until you reach the pedestrian crossing. Try to go west from here and Lillie will block your passage. North it is then! At the far end is the entrance to Malie Garden, where Professor Kukui is waiting. You do remember, right?