Elder Scrolls 3: Morrowind lets you fly around, jump over whole cities, and kill most NPCs with one high powered spell. It's a very unique game, but if it clicks with you there's nothing quite like it. Vibrant modding scene too.
The base game is certainly playable, but if you want modern resolutions and some vanilla-friendly bug fixes OpenMW is an open source recreation of the engine, there's instructions on their website.
Check out Tamriel Rebuilt too! It's a mod to add the mainland that, while incomplete, already has 100s of quests. Good quality control too.