In this paper we address the problem of increasing the flexibility of the contemporary wireless sensor and actuator networks (WSANs) in regard to the design of the nodes. For this we propose the concept of a Plug&Play enabled modular WSAN node platform. According to our concept, the new WSAN nodes with desired functionalities can be built by stacking together the different hardware modules encapsulating power sources, processing units, wired and wireless transceivers, sensors and actuators, or even sets of these. Once a node is built, it automatically discovers and identifies all the connected hardware modules, obtains required software and tunes its own operation taking into account the node's structure, available resources and active applications. In this paper we first present the concept and then report the developed hardware and software architectures and the most critical mechanisms enabling implementation. Also we discuss the practical implementations and report the evaluation results for the prototyped solution. Our results show that the developed platform is much more feature and resource rich than the existing ones, which is achieved at a cost of increased consumption and size. We believe that the unique features of the proposed platform have the potential to drastically change the procedure of WSAN development, especially when it comes to experimenting and developing dynamic WSANs with a heterogeneous structure. In this respect the hardware identification and reconfiguration capabilities conceived in the platform can be utilized in full and may drastically increase the performance of WSANs, if combined with novel control and optimization schemes yet to be developed.