Desaru Fruit Farm
Sungai cemaran desaru, tel. +60-7-822 5886, (http://www.desaruff.com/). 100 cultivars of fruit and a mini zoo.
Ostrich Wonderland
Emr 15 teluk ramunia, tel. +60-20-78265846, (http://www.geocities.com/...). 300 ostriches rides for kids available plus eggs, jerky, leather products and more. open 10 am-6 pm daily. rm 10/6 adult/child.
Desaru is not a well developed resort, but there are a lot of things to do while you are there. Take advantage of the nice wide beach, and of the shallow and clear water. All 22 km of it is open to all, even the stretches in front of the hotels, but most locals congregate at the public beach, which provides a carpark, a few barbeque pits and grotty toilets.
Snorkeling is a good option, but don't expect banks of colorful fishes and wonderful sea creatures. Scuba diving here is out of the question, although some of the resorts do arrange trips out to Johor's northern islands.
One thing you can definitely try is surfing. If wind is good, the waves are quite OK, and especially the lack of jellyfish and sharks makes it an attractive location. During December, January and first half of February monsoon season, the waves are almost constantly high, but do expect the water to be a little cold. There is a company that rents surf boards for 20 RM/h and provides surf lessons for 100 RM. The same company rents jetskis.
Excursions to see the fireflies api-api in the Sungei Lebam river are very popular. While theoretically possible to arrange yourself, this would require bumping about dirt roads to find the dock and then cruising around a pitch-black jungle river, so it's much easier to join one of the hotel excursions. The best time to go is after rain on a new moon.
Other options mostly of interest to families with young kids to entertain are: