Route 115 is actually pretty featureless, at least coming from the Rustboro side. Go north and up the ledges for a bit, then west and down to the beach. Run north and east along it to find a Zinc in the sand next to a boulder. Zinc is an item that boosts your Sp. Def. Evs by 10.
The boost is barely noticeable if you use it right now, but it’s more influential (a 2-3 point boost) by Level 100. It can also - and honestly probably should - be sold for $4,900 as you just as easily earn EVs through battles or through Super Training. Route 115 is basically void aside from that.