I love the Vita! Ok so!
That 16gb is going to fill up rapidly. I had a 16gb since around launch and recently went all in and bought a 64gb card.
As for cases, there are some official "hard" cases that I swear by. They're heavy but it means more protection. Given how delicate the analogue sticks are I'd suggest that over a soft case.
As for games my favourites have been;
Luftrausers - Simple, enjoyable "space" shooter (but isn't in space).
Duke Nukem 3D - Port of the original with loads of expansion packs included.
Persona 4 - Huge JRPG, don't normally like these games but there's a dungeon exploring side to it that I really like.
Street Fighter X Tekken - basically a Street Fighter game but with loads more fighters.
Geometry Wars - a pretty good take on the modern classic, the new modes are a bit too much but there is a Classic mode buried in there.
Zen Pinball 2 - DLC is cross compatible with the PS3 so if you already own that you can play the same tables on the go.
Motorstorm RC - simple launch title remote control car game, reminds me loads of Micro Machines. Very playable, loads of content, easy to waste hours in.
(I'll come back to this)