Look for Papiosaur's "Pack Ultimate". It will install and setup many games and other programs for you and give you something to play with. In addition, it also will put a cool background up for your desktop and setup a dock for you to use. It is a great effort to install and setup things for the lazy, or uninformed users.
What model of G4 MacMini do you have? Remember, if it is not the 1.5GHz model, it will only have 32mb of VRAM and you should probably turn off the enhanced display to save your Video RAM from running low in certain circumstances. The great thing is that you can enjoy playing around with it for 30 minutes at a time between re-booting to find out if it is something you like and want to buy. Then, if you want something better you can look for a G4 PowerMac (single CPU is okay as MorphOS currently can only use one CPU), so you can put better graphics cards and a sound card in it for better performance. Plus there are third party accelerators of 1.6GHz, 1.8GHz and 2.0GHz that you can use, if you can find them.
Enjoy, and welcome to Amiga.org!
Edit: One last thing you might want to consider before you make a purchase of MorphOS2.x license key, is that it cannot be transferred from one system to another, as it is hard-coded to the MAC address of your computer, so make sure you have the MorphOS2.x compatible system you really want before you buy a license key for it. Also, the MorphOS Dev. Team is currently working on support for the G4 PowerBooks that have USB keyboard and trackpad interfaces. Generally speaking, this means only the last couple of G4 models will be supported when this port is completed. It is unknown when support for any G4 PowerBooks will be completed, so if you are happy with MorphOS2.7 and want to use it now without the time restriction, don't wait for PowerBook support to come, as it might be a few weeks, or it could be many months.