Alright, since im a lazy ass so im just gonna do a list i do whenver im bored.
1. Play on LAN or server. so you will have somebody to talk to and play with
2. Make some redstone creations
3. Play adventure maps
4. Try to play like some famous minecrafters and talk the same way they talk. Pretty fun but dont make a video of it you might get haters
6. Play on hardcore mode with a village seed. try to pick a house on the village and defend it.
...which is why I would suggest getting a mod for better terrain. The ones that emulate 1.7.3 are good. There's also BWG but I have a feeling it doesn't fix the problems it has (it doesn't remove EH).
If you don't care about terrain, go get other mods. Or play Dwarf Fortress, which is similar but with multiple characters (you control a fortress of Steves instead of one Steve) much harder, much more complex, much more fun (if you can get past the control scheme, ASCII, and difficulty curve brick wall), and much more Fun.
I'd say build bigger then. I recently started growing bored so I decided to build a Terran base from StarCraft 2.
I ended up with the building being to large to fit within the height limit when I tried that, if I got around to actually finishing the building it most likely wouldn't fit within the render distance either.
Well, if it's the calling of the previous version you're feeling, downgrade. If it's the terrain, get a mod. If you have done everything there is to do, download some mods. If you still aren't satisfied, i'm sorry to see you go.