If you punch a chest a few times, it'll become a carryable item, but it also loses its contents at the same time, so you need to carry those separately.
You can also edit your skin, to (possibly) give the illusion of looking like, well, anyone you like, I guess (if you squint and have a good imagination), but you can't change the character's shape.
There's no tumbleweed in the game, but it might be a cool addition in desert or wasteland biomes - would allow for building walls that look like they've got barbed wire along the top.
I haven't played in ages, but I have a feeling I'd end up going around irl thinking "hm, how much iron do I need to move this Tube line through my flat?" :C
Looks like there is an online-mode setting that controls regular auth checking.
So, with that turned off, it's just a case of setting up an authenticating proxy server in front of the Minecraft server which does the login checks (against Teh), and it's as simple as that?