It's amazing what can be done to a standard shipping container. Here's a partial list of common modification to give you some ideas:

• Rolling doors
• Swinging doors
• Sliding doors
• Standard entrance doors
• Windows
• Electical set up
• Plumbing (sink, bathroom, etc.)
• Tile flooring
• Dry wall
• Air conditioning
• Partitions
• Vents

Other modifcations include:

• Combining containers for military or fireman training exercises
• Stacking and adding a roof for a warehouse
• Joining containers 2 or 3 wide
• Ticket booth
• Soundproof recording studio
• Tradeshow booth
• Temporary housing