ive finished the thieves guild quests and i was just wandering if anyone has any suggestions of places that have lots of good items to steal and it is actually a challenge to steal them

I like stealing from the castles. My favorite thing to do is to complete someone's quest to collect an item, let them have it and put it away and then immediately steal it back. Gotta be careful because sometimes you can't put it down again because it is a quest item.