About an hour's drive northwest of Kuala Lumpur, you'll find an off-the-beaten-path retreat from the sensory volume of the big city called Kuala Selangor. Located at the entry point of the Selangor River into the Malacca Strait, this small town is approximate to some interesting adventure activities, with the ocean at its doorstep and both the Kuala Selangor Nature Park and Bukit Melawati at its wings.
If you're near the ocean, you may be thinking water-based activities. Although not an area particularly known for clean, attractive beaches, Batu Laut and Bagan Lalang are passable options for sun and swimming. If you're making moves toward Kuala Selangor, then you'll need to climb to the top of Bukit Melawati for incredible views of the ocean (best at sunset) and the nearby nature park, as well as remnants of the fort that had guarded the entrance to the Selangor since the 19th century. Take the tram up the hill for a nominal fee on the weekends. You may also explore the mangrove forest and wetlands of the nature park, although there are some issues with maintenance and accessibility as of 2012.
Better yet, wait until nightfall, when the Firefly Park at Bukit Belambing is in full swing. For 4 RM (1.25 USD) a person, boats take you down the river to view the incredible synchronized flashing of fireflies in the trees along the banks.