My thief/assassin just became Master Thief and has the skeleton key. Any chests anywhere I should go open with the skeleton key? Im looking for light armor, short swords, bows, ect.

Each of the three Houses has a vault in their canton in Vivec. Lots of stuff to sneak in and steal.
There's also a similar room at Ghostgate.

From memory, daedric dungeons usually seem to have the high-level locked chests.

Solstheim has some locked places worth visiting, specifically Gyldenhul Barrow.

Thanks. I'll check those places out.

Typically, if I had a character who played all the way through the Thieves Guild to become Master Thief, my Security skills were so awesome that with some Master Lockpicks, the Skeleton Key became relegated to one of the many priceless treasures strewn about my Stronghold/Manor/Cave, lol

from memory, venim tomb close to the zainab camp have a lock around 90, is some loot in there, ebony staff,a unique bow etc.
it is lgnpc-quest related though,so I wouldn′t go there prematurely with lgnpc installed.

If I remember correctly, you only have to have a security of skill of about 50 with decent lockpicks to be Master Thief. If you grind through the quests your security won't shoot as quickly as if you had been dungeon raiding and picking locks for most of your time.

After a dozen lockpicking characters, I finally realized the simplicity, power, and efficiency of home-brewed restore magicka potions combined with Alteration. No picks required. But there's just something elegant about picking a lock, eh? I always enjoy breaking open the chests stationed around statues in Daedric shrines. Talk about phat lute.