I would stick to one activity for as long as your LO is happy and stimulated - switch to something else if he is fussy and needs distracting. If OT/OS has been an issue, stick to low-key activities, and it is generally a good idea while they are so young to keep to low-key activities for the 15-20 mins before nap time. Low key activities are things which DON"T involve toys/books/tv/etc - so perhaps listening to mummy or daddy singing a few soothing songs, having a walk around the house in someone's arms, going for a short walk outside in a sling, laying on the bed with mum or dad and making faces at each other.
HTH
anna
x