Depends on your interests. Game mods are pretty easy to find and join, because you can do that using the internet. There are also a few sites where you might look for short film projects. It might not be useful to work on sound libraries though, unless you're particularly interested in them or have ideas for something extraordinary, because you'd be spending ...
Apart from strictly "audio production work":
• Selling sound effects and sound effects or sample libraries
• Preset design for musical instrument or audio effects plug-in developers
• Marketing for musical instrument and audio gear manufacturers
• Renting recording gear and studio facilities
• Acoustical consultation and studio design/building (but you need ...