I have thought about doing the outside time during naptime too but in an emergency when you hear/see something on the monitor that needs your attention a lot of time will be wasted - maybe too much time getting the other kids rounded up to go back into the house to deal with the younger child that it just never seemed safe to me.

Is it new activities you need or new rules for the old activities - volume of voice during naptime. They can still play with the cars but they need to keep their voices down. Banning things like active games, running, balls, etc. makes sense of course but that is a long time to make 3 year olds be still and quiet doing sit down type work.