Fifteen years ago, Langkawi was just a backwater island supporting small fishing communities. When the government came in with big money to develop the place for tourism, they thought they needed a catch, so they dug up some old moldy "legends" about the island and have tried to market them as bona fide cultural attractions. Basically, any of these sorts of attractions are more hype than anything else. If you want to experience culture, take a ferry to Penang for an overnight in historic Georgetown. Now that's something to see.
In terms of beaches and watersports, most resorts are self-contained units, offering their own equipment rentals and planning their own outings.
Outside of your resort, there's some fairly decent diving to be had. Asian Overland (tel. 04/955-2002; www.asianoverland.com.my) can arrange day trips with two dives to Payar Marine Park within Langkawi's extensive island network. They charge RM280 ($78/£39). You can also snorkel for the day for RM160 ($45/£22) per person. There's an interesting snorkel attraction off Langkawi -- a platform in the middle of the sea that floats above a coral reef. Day trips to the platform include rides in a glass-bottomed boat, snorkeling, and lunch on the platform. It's an all-day affair for RM230 ($64/£32) per person, starting at 8am and getting you back to your resort just before dinnertime.
Asian Overland also plans round-island boat trips to "island-hop" at beaches and into mangrove swamps (interesting) with a stop at the Pregnant Maiden Lake (one of the before-mentioned over-hyped places). See if you can get them to skip the lake and take you to the Batik Art Village instead. They'll cater your tour so you can see anything you want.
Perhaps one of the loveliest additions to Langkawi's attractions is the Ibrahim Hussein Museum and Cultural Foundation, Pasir Tengkorak, Jalan Datai (tel. 04/959-4669). The artistic devotion of the foundation's namesake fueled the creation of this enchanting modern space designed to showcase Malaysia's contribution to the international fine-arts scene. If you can pull yourself from the beach for any one activity in Langkawi, this is the one I recommend. Mr. Hussein has created a museum worthy of international attention. Truly a gem. It's open Saturday through Thursday from 10am to 6pm; adults pay RM12 ($3.40/£1.70), children visit for free.