trekking
The mountains directly behind the city have decent hiking trails. The area is scenic and not too crowded, and you can get a good view of Makarska if you hike high enough. Be sure to stay on the trails and be aware that there is one species of poisonous snake that lives in the area.
rafting and kayaking
About a half-hour drive from Makarska, you can go rafting or kayaking along the Cetina River. The river is generally calm, although there are a few fast places, and one section is particularly dangerous. There is a tour agency that offers guided trips down the river. If you decide to go, look for the cave that was used by Marshal Tito's Partisans to hide from the Nazis during World War II.