Despite being best known as a diving and laying around on the beach destination, both the Perhentian islands have some decent (and not-so-decent) forest walks that will appeal to those looking for something a little less sandy to do.
It's possible to walk almost the entire way around Perhentian Kecil using d'Lagoon as a starting and finishing point, while on Perhentian Besar you can do a loop on foot to all the developed beaches save Bubbles (which is only reachable by boat). The trail on Perhentian Besar is considerably better marked and maintained than on Kecil, but on both islands be sure to take sufficient water, a hat and sunscreen.
Aside from tramping through the forest, you'll see plenty of birdlife and monkeys -- the latter particularly near d'Lagoon where there is a bunch of macaques (including one that is prone to psychotic episodes on Turtle beach) that you'll see, or hear as they munch on fresh mangoes, while between West beach and Tuna Bay there is a very playful chilled out family of dusky langurs -- we saw them a few times on the roof of the Turtle Bay Divers office at the southern end of West beach.
Monitor lizards are everywhere on both islands and some resorts consider them pests as they tend to get into the garbage -- note to resorts: perhaps consider better waste management policies? Some of the lizards are quite small, but in the lagoon behind Teluk Dalam on Perhentian Kecil we saw a couple well over a metre in length. They'll often hear you and bolt before you see them, but don't be surprised if you stumble upon a few.
On Perhentian Kecil, a trail leads from d'Lagoon first to Turtle beach (10 minutes), then Adam & Eve beach (20 minutes) and then eventually to the wind turbines (one hour in total). The walk this far takes about an hour. From there you can continue onwards to Long beach (we didn't walk this leg). From Long beach there is a separate trail that leads to Coral Bay and takes just 10 minutes. From Coral Bay, walk south, past Senja and Butterfly, and just keep going. It will lead you eventually to Keranji (30-45 minutes), then Petani beach (1 hour from Coral Bay) and eventually to the village (another 30 minutes). From the village you can continue north on a paved trail, but you'll reach a point where the dirt track heads left and the pavers go straight ahead. Turn left and take the dirt trail – we took the pavers, which stopped suddenly, and then tried to bush bash our way to Long beach. We ended up lost in the woods for two hours and it was not much fun at all. The dirt track though apparently leads to Long beach.
On Perhentian Besar, a rough trail starts at Mama's and continues north, running parallel to the beach and then joining a trail from PIR after about 15 minutes. It then turns and continues south through some good forest and then tall grasses to deposit you at the centre of Teluk Dalam 30 minutes later. From there, walk west along the beach and there is another trail (we didn't try this one) which leads over the headland to Teluk KK from where it is a straightforward path to Tuna Bay. Where you reach Coconut Chalet, the trail continues for about 15 minutes behind the first row of bungalows on the headland to take you to West beach, leaving you by the Turtle Bay Divers office.