There is absolutely nothing to see or do in Kuala Tembeling, but you have to pass through if you want to get to the Taman Negara national park by boat. From Kuala Lumpur, a bus takes you to Jerantut. From there, you can either get to the park by bus or by boat, but you have to take a shuttle to Kuala Tembeling.
The boat journey is really worthwhile - it takes around two and a half to three hours on the River Tembeling which leads you straight to the entrance of the park. The boats leave at 9am and 2pm during the week, but on Fridays there is just one boat which leaves at 2:45pm.
At Kuala Tahan, you will find much more facilities, shops, restaurants and accommodation. I particularly liked the boat-restaurants (even though they didn't sell beer).