My 3 year old DS attends a Montessori pre-school 2 mornings a week and tolerates/enjoys being with the 5 year olds (at times he describes them as silly) - given a choice he would prefer to stay at home. There is plenty of scope for academic challenge on the days he feels like it, and there is the sandpit etc for the days he doesn't. Not sure what will happen next year though when the 5 year olds head off to school...