Something I like to do to keep myself entertained/busy ingame (written as a list):
* Play SP (single player) as if I'm designing the world/s for others, and stick to a theme. For example:
* Use dimension making mods (mystcraft/rftools dimensions etc) and create a diverse set of worlds.
* Make each world have a noticable "culture" (ie; architecture style, implementation of different mods across worlds)
* Example of above: An RF tech world, a EU tech world, a magic world all as separate worlds/dimensions.
* Challenge a creeper to a Greco-Roman wrestling match. Win.
* Make diverse farms in each world, each different according to culture.
* Pick inspiration from various cultures in RL or Sci-Fi/Fantasy to do so if needed.
* Create lore ingame which evolves as the worlds get fleshed out, written in ingame books.
* Create a void age/dimension(s), build huge "spacecraft" of various ingame cultures.
Not a long list, but that will keep you busy.