The waterfall-strewn Namtok Samlan National Park is located 115 km northeast of Bangkok, making it a popular weekend destination for capital residents in search of some natural respite. Though its highest peak (Khao Khrok) tops out at a modest 329 meters (1,080 ft), the panoramic views overlooking Saraburi Province from the summit are a fantastic sight to take in, and well worth the hike.
If water is more your thing, you've come to the right place. Aside from the availability of canoeing and kayaking on a selection of scenic reservoirs (including Khaoruak), visitors can trek 2-3 hours (on average) in order to visit a spectacular array of seasonal waterfalls that include Pho Hin Dat and the park's 3-tiered namesake Sam Lan. Don't be surprised if you catch a glimpse of some monkeys, wild boar, or any number of exotic birds/butterflies on the way.
Like most national parks in Thailand, camping is permissible both in tents and bungalows provided you make reservations in advance. Check out the official link for more information.