Display cases

Display cases are a Lockpickable container usually including a weapon with them. They may sometimes have a Skill Book, possibly about the weapon's style (e.g. Archery Skill Book for bows, Two-Handed for greatswords etc.)

Display cases are a good way to level Lockpicking, because they often are Adept-level and above. For example, The Companions' living quarters have many display cases, with one which has an Archery skill book and a bow. Every display case is Expert-leveled in there, giving a good amount of XP.

In the Silus Vesuius's museum, there is a display case which can't be unlocked by lockpicking. It has a part of Mehrunes' Razor in it, and after completing the quests for Silus and choosing to save him in the final quest, the whole Mehrunes' Razor. Although, if you use a Shout, the part/Razor may glitch out of the case, allowing you to receive it.