You can start with Implementation of Rounting Protocols, Distribute Scheduling Algorithms, clock synchronization protocols, and distributed key management for wireless sensor network in NS2.

Take a reaserch paper from any of the above mentioned area and implement that paper in NS2 to confirm the claimed performance by the other.

NS-2 is good, but it is difficult to use for a beginner.

So ,I am trying to find other tool.

"A survey of simulators, emulators and testbeds for wireless sensor networks", this paper will be helpful.