Today we are rather particular about Optional Prayers but ignore the Compulsory and Obligatory (wajib). The Doctrine (shariat) wants your worship to be beautiful. The Compulsory Prayers (faraiz) are completed by the Optinal Prayers (nawafil). The latter are not substitutes of the former. Having said the Morning Prayers wait for sunrise and then say the four units Awwabin Prayers. Sometime later you can say the Chasht Prayers, four or six units, in fact the number may go up to twelve.
Similarly if you have missed your fasts then there is a chance through out the year to compensate for that. Each month has got some special fasting occasions that can be availed.
(Sitting on Monday, September 27, 2010)