Obviously, if the chapel will also be used for daily/intimate Masses, there are certain requirements that must be followed and they drive the process. However, if it to be only an Adoration Room, simplicity is acceptable with certain minimum requirements including:
Place to properly display the Eucharist. This can be a table for a Monstrance or Tabernacle. I know of several that have Tabernacles that have little doors that open and close such that the person can open the doors that display the Eucharist when they are there and close them upon leaving to facilitate sporadic/non-perpetual adoration.
A candle that gives notice of the presence of the Eucharist in the room. If you have the “doored Tabernacle”, you should have additional candles that are lit prior to opening the doors and extinguished after closing the doors.
Appropriate kneelers for the worshippers.
If the room is small and intimate, IMHO there should be minimal subordinate decorations as in Sacred Art or statues of the saints or Blessed Mother. I don’t know how appropriate it would be for people to come to the room for a Marian devotion when they ahve the ability to expose Christ.