iOS 3 "Continues to Live"
There is a 3rd-party firmware called "Whited00r" which is technically a pre-jailbroken iOS 3 already modified to look like iOS 7 with a bunch of the extra "cruft" removed to be optimized for performance. I won't link to it here, it case that is not allowed.
It isn't going to win any speed competitions but it seems to make first-gen devices much more usable!
I don't think the iPad can be downgraded from 5.x back to 3.x without the "SHSH blobs" for it.
If the iPad is jailbroken, here are some
tweaks