Starting things off, battle the Nurse straight ahead. It helps a lot if you're going to be scouring this desert for items and the like. There is a sandstorm going on in every battle, and your Pokémon will have their health taken down to low levels. Straight up from the nurse is a guy in sunglasses and a suit, and he will give you Soft Sand, a held item that raises the power of Ground type moves. Going to the far right and down is a small pathway with a female hiker. Going further down is a small area with seemingly nothing in it. Go to the small square and press A facing the middle and you will get a Fire Stone. If you have a Growlithe, don't use it yet, because you will miss some of the great moves it gets.
At the top middle is where you will find the Relic Castle entrance. However, behind it is a Heart Scale, and 5 spaces south and 4 spaces west of the entrance is a hidden Ground Gem, which is a consumable item that boosts a Ground type move by 50%. Going left from the Heart Scale, you will find a Nugget, worth 5000 dollars. Which is always nice. Other items in here include shards and Stardust, but there are numerous of them strewn throughout the entire resort.
In terms of new Pokémon, you can find Sandshrew, Dwebble, Scraggy, and Maractus. Dwebble and Scraggy were both seen before, and both are good in their own ways. If you liked what you saw from Burgh and Team Plasma, respectively, then you may like these two.
Sandshrew is a long time favorite Ground type. It is not too fast, but it's physically defensive and offensive. It lacks some power when it evolves, but it is certainly usable. It got buffed from Black and White to Black 2 and White 2 since now it learns Earthquake and Magnitude through level up.
Maractus is quite the interesting Pokémon. It doesn't have too much going for it, being a more jolly Cacturne. It doesn't get the moves it really needs until X and Y or through breeding in these games, so Maractus is really missing the tools it needs to be a good Pokémon. If you plan to use it, it will be quite difficult.
Heading inside of Relic Castle means we're done with the sandstorm for now.