While I believe the Alchemy Building Proposal is a far better option, how about this additionally:
Under rare conditions, a “box” item is given that allows the player to add to his/her inventory a single rare ascension item (telescope, darts, rings, etc) of his/her choosing. This would be a perfect prize for the advanced levels of the monthly events.
It would work better as an item to be activated instead of being opened immediately upon receiving it. In either case, though, the number of each ascension item that you own already should be included, so that you don’t accidentally pick the wrong item instead of what you really needed.