Now you are free to do what you please. You can hunt to get some skins for crafting as well completing side missions, activities and finding collectibles to earn money and exp. Once ready, head to the marked location. You may want to activate the nearby radio towers first so you can have a much better view of the area.
After clearing the map, go uphill to Dr. Earnhardt's Mansion. Enter the shack outside for a scene. After talking to the doctor, check Daisy upstairs.
Use the glider outside the Dr's house and glide to the marked location north. The cave is underwater so dive and swim fast until you reach the other end of the cavern. Continue until you find a long drop. Jump down and swim towards the vines to climb up. Continue following the path until Jason gets an acid trip. After the trip, grab the mushroom then exit the cave. Make your way to the Greenhouse for a scene. After that, go upstairs to check Daisy again and to complete the mission.