Is there a way to simply regain spells used up without the necessity to rest? I seem to remember there being a spell or something like that that would do the trick, just cannot seem to find it.
You might have been thinking of the pearl of power, which is in the DMG. There is one for each level of spell and can be used to recover a cast spell of that level once per day.
Rary's Mnemonic Enhancer (and later Mordenkainen's Penultimate Cogitation) can do what you are asking for, in limited ways. Rary's spell is still around, I believe. Mordenkainen's spell existed in 1E but might have slid into some 2E book.
A spell that allows you to sleep the equivalent of 8 hours in 1 hour would be cool. Enter a pocket dimension or alter time/space. Make it at least a 3rd level spell whose slot isn't recovered when you finish your rest.