The park has a single trail ascending through three camps to the summit of Mt Marojejy. The trail officially begins at the park boundary, about a 1½-hour walk from the park office at Manantenina. If you want to cut down your trekking time, you can arrange for a ride to the trailhead, but it is a beautiful walk to get there, along a dirt road through lush mountains, small villages, and rice paddies.
Camp One (Mantella), at 450m, takes about two hours to reach, and has six cabins (wooden frames with canvas walls) and a campground. This is an area of lowland rainforest.
Camp Two (Marojejia), at 775m, takes an additional two hours and a bit more work to reach. It has four cabins with fully made beds, and a large covered kitchen and dining area. This is a transition area between lowland and montane rainforest, and is the best place to see wildlife, including colourful millipedes the length of your hand (a lot more enjoyable than it sounds), leaf-tailed geckos, and paradise flycatchers. It is also a wonderful place to hang out, surrounded by the sounds of the forest and the rush of a nearby stream.
Camp Three (Simpona), at 1250m, is the base camp for ascents to the summit, and has two cabins with a sheltered dining area. It is a very steep and strenuous climb to get there, requiring both hands and feet as you surmount one root after another, a challenge magnified when it's wet. The final leg to the summit, 2132m high, stretches 2km and can take up to four or five hours to traverse.
Note: the trek from the park boundary to the second camp is a nature expedition. The trail from Camp Two is a climbing expedition. If you prefer the former, there is no need to go past Camp Marojejia, which anyone in decent shape can reach. Beyond it you must be very fit, and prepared for cold weather.