I found inventory system a good start point for any kind of game. The existing inventory systems are lacking extendability. Therefore I designed my own inventory system to be more programmable:

• Item as an abstract base class for easy extension
• Model-View-Controller paradigm for easy control over different inventories
• Tree data structures for data storage and manipulation of the inventory
• provide customizable tool-tip display
• Animation and sound interfaces for user interaction
• Game update loop encapsulated and broadcasted to all items
• Callback style item usage definition