Place the cedar plank in a shallow pan and cover with orange juice or water; soak at least 30 minutes, or 2–4 hours for best results.
In a small bowl combine the brown sugar, paprika, cayenne, garlic powder and dried thyme; season with salt and pepper and mix well.
Preheat your gas or charcoal grill to medium heat (about 350°F / 175°C).
Pat the salmon dry and place it centered on the soaked cedar plank so no fish hangs over the edge.
Sprinkle the spice mixture evenly over the top of the salmon, then rub the fillet lightly with olive oil.
Set the cedar plank with the salmon on the preheated grill, close the lid, and cook 12–15 minutes, or until the salmon is uniformly pink in the center and flakes easily with a fork.
Using a large spatula, transfer the plank and salmon to a flat surface and let rest briefly while you prepare the salsa.
In a large bowl, combine the diced watermelon, diced mango or pineapple, chopped jalapeño, chopped cilantro, lime juice and honey; gently toss to combine.
Add the crumbled feta to the salsa and toss once more, being careful not to break up the feta too much.
Spoon the watermelon feta salsa over slices of the rested salmon and serve immediately.