For an amazingly cool item that is utterly useless in combat, please consider the Rod of Security.

Rod Of Security - In A Nutshell

You and your friends get a sum total of 199 person-days of vacation time in paradise. Once you are done (or kicked out) you must wait 10 days to reuse it again, regardless of how long your stay.

“Visitors can remain in the paradise for up to 200 days divided by the number of creatures present (round down).”

You get all the food & water you need - and you do not age whilst inside. Points of interest: time passes normally for those ‘outside’ your staff’s secure domain so you may miss out on a lot of worldly events. Also, the staff remains behind, presumably unattended wherever you last touched it. When going in, it isn't clear how much non-living matter you can take with you ('whatever is on your body' or 'your whole estate'?). It also is not clear how much you can do with the created paradise in terms of study, training, research, practice or other more work-oriented progress. Can you create a luxurious smithy? An amazingly well stocked library for the more geeky-introverted sorts? Can the collective paradise have bits of a carnival plus a dash of bathhouse? What sorts of servants are in there - what forms of support do they provide? So many questions.

Home Away From Home: Possible Uses

Of course, those hiding in this staff live much longer. One person gains 199 days 'in' for each 10 in their real world, giving about twenty times longer lifespan. Given two such staves a group of nearly twenty folks could skip back and forth between safety zones (needing only seconds of ‘real’ world time). Pregnancy would endure for centuries, children would never age. Imagine ten thousand years of puberty.

The good news: wizards, given enough stuff (rare books, unique components &/or dedicated materials) could produce, research and develop any spell, formula, magical item or McGuffin they wanted to develop - free of interference or even need of servants. One may need an overflowing Tenser's Disk going in just for your stuff.

Story ideas galore:

• You meet &/or you are people from these Immortal Gardens that got kicked out. How old are you? What is your story in that time? What great events did you miss? How do you see present cultures of your brand-new ‘real world’ of 5e D&D?
• An ancient lich, dragon or other long-lived entity has lost track of his enemy, hidden somewhere on the material plane. You must find a way to track this person (or group) down via ancient legends. Unbeknownst to you, you seek a very well hidden staff in a tiny cave-nook... in one of many, many mountains.
• The locals need the help of an ancient and powerful hero (pick a class). They reached an age well past retirement and they have been slowly eating up the last of their biological days in this staff. You must convince them to abandon their amazing & ultra safe retirement to join you and your party on some quest. They would, no doubt, know much ancient and vital information from ages long past / passed.
• The wielder wants to use this staff on the deck of a ship and sneak 20+ giants into an enemy kingdom’s port when the safety-paradise is ended. The ship hits some snags - there is a great danger of twenty giants appearing on deck.
• A master thief sneaks into the king’s or dragon’s hoard and takes a vast amount of loot and hides inside this staff, having no other means to escape. The rogue left this staff amid a pile of branches, torches and other sticks - no one suspects. That said, the rogue MUST return to the crime scene within 199 days.
• An epic dragon, losing the fight, has its tail touching this device - disappears for 1 to 199 days. Perhaps the dragon returns the next day to surprise the dragon 'slayers'? Perhaps he or she waits for half a year?
• The siege is lost, the enemy has stormed the gates / is on their way. Now, 199 people can hide, but only for one day. Where do they all gather so the next day they have the best chance of successful escape? Who is left behind?
• A wizard performs his entire school inside two or more of these devices. As he has multiple staves young kids go in and train only to come out masters that are still young kids years later.