New software will let you run a webserver on it, or connect to OSX with the new connection utilities produced by the Newton community. All that is easy enough if you know what you are doing.
In terms of older software, you could do something like set up Notes with custom stationery to do something specific to your world. There are some reasonably easy ways of doing that if you go looking and experiment. Beyond that, tools like Newtbasic + Newtcard especially were really cool, and can still be purchased (or could about seven or eight years back). I gather that there are other things like a wiki available (haven't tried it myself).
One of the tragedies of the Newton these days, however, is that most of the third party software you see on UNNA is orphaned, with no serials available and dead links to the companies concerned. This really limits what you can do -- though, having said that, there is enough to make things interesting, so don't abandon all hope.
Now, if you happened to be a programmer, you could do custom software with the old tools that are now free, but that back in the day cost thousands. That stuff is all on UNNA. I gather that there are also tools for doing Newton work in modern programming environments, but as I am no programmer, I know little about either.