If you are working, even with a child as young as three to six months, then reading a book to them is a pleasant activity which does not require any active response from the child. Starting this kind of activity at an early age is good because it encourages use of language and recognition of simple objects. Even reading nursery rhythms is useful as the learning of cadence, rhythm and phrasing helps with language development. Listening to music which is quiet and soothing without any sudden loud phrases or jarring noise, (this is surely a matter of personal taste) can be soothing to the child. In one so small being held comfortably and patted slowly or stroked gently can also be soothing.
Quiet play does not include computer games or games which require interaction at a high level. These undoubtedly have their place but it is not as a pre cursor to bedtime. It is important to stimulate immagination but bed time is not a good time for this particularly if your child has nightmares or fears of the dark or being alone.
Quiet time is a useful time in bonding with a parent who is always busy, or not available during the day. The last hour of wake time is a good time to spend listening to your child telling you about his/her day, enjoying close physical contact, and learning to relax and wind down together is good for both parent and child. The last hour before bed time is quality time for parents and children and we do all a disservice if we just switch on the television and expect that to be enough.