The Rod of Security gives a person access to ‘any paradise one could imagine’, specifically:
‘… the rod then instantly transports you and up to 199 other willing creatures you can see to a paradise that exists in an extraplanar space. You choose the form that the paradise takes. It could be a tranquil garden, lovely glade, cheery tavern, immense palace, tropical island, fantastic carnival, or whatever else you can imagine.’
This ageless paradise has a 199 day limit, divided by the number of occupants staying within Example: a party of five gets ( 199 days / 5 people = ) 39.8 days maximum. Once out it takes ten days to regain this 199-day-persons capacity.
Plenty of rest, good times and amazing food (i.e. ‘possible weight-gain hazard’) – but so much free time… with incredible resources! Imagine gaining new tool proficiencies, catching up on some downtime stuff or whatever you like. But could one research old &/or new spells? Develop a formula / blueprint / pattern for almost any magic item? Research ancient lore in a paradise-library? Suddenly this rod is a very powerful item. Possibly derailing a campaign – or even presenting as a bit too powerful?
Given access to ‘any paradise’ – be that training &/or library-research – what limits exist for players’ gains in skills, abilities &/or knowledge?