Minecraft is basically the sandbox of sandboxes. If you want it to consistently hold your interest, I have a few suggestions:
1) Play survival, and on the hardest difficulty. Playing creative may allow you to build large-scale projects, but every block you get in survival is earned. Makes you appreciate things.
The monsters are overwhelming at first, but usually become trivial once you get your base figured out, and start getting armor and sophosticated weapons. But still, sometimes despite your best planning, you can find yourself in a dangerous situation... Usually involving creepers. Again - - helps with perspective.
2) Play on a multiplayer server. Find a good community server on minecraftforum.net or host one yourself. Play with friends. It always motivates me to know that somebody could appreciate my work (even if they never do).
Try to get a copy of the world file from the sever admin from time to time. This allows you a sense of permanence, so that you feel justified in making epic-scale projects.
3) Have goals and projects. Pretty self explanatory...a game as open ended as this could leave you meandering without a focus. For me this means establishing a base with all the tools I need to make things (my own "laboratory"...like dexter's lab), then branching out into landscaping, farming, or exploring.
4) Try mods. Namely tekkit. Tekkit makes the game more interesting by adding more blocks, resources, and crafted goods & machines. Tekkit light is more balanced than Tekkit classic. Regular tekkit apparently allows you to go to the moon now; haven't tried it yet.