Having actual unlocks for achievements is a very good idea and I think I might go for a similar route to how Frictional Games did it.
There's a password protected "secrets.zip" in at least two of their released games which you unlock the password for in-game, then there's a bunch of extra goodies in there.
So I think I'm going to do that for some achievements, have a few locked zips included with the game which you then get the password for when unlocking certain achievements.
Sure, once someone knows the password, it's available everywhere on the internet but that's kinda whatever I guess, everything will be available on the internet anyway.
So that's one thing I'm probably going to do.
As for skins, this would have to be put way on the back-burner for now, as this would add loads of additional workload.
Stat boosts and such will probably not happen this I know already.
I'll be looking into more ways to reward the player for achievements in the future, some of them will stay as bragging rights though since I can't have rewards for every single one of them.
Thanks for the suggestions/feedback. :)