If it is a time issue, as MON is suggesting - one thing too is to maybe use a timer. I do this with DS when it is play time, because I can say "we have only 10 minutes to play/build" but when the time is up, he does not think 10 minutes has already passed... but with a timer, he knows how much time has passed, and when the timer buzzes, there is usually no confrontation about moving to the next activity.
For things like books at the table (our kids like to bring toys), one thing I recall seeing at a school that I thought was neat - they had a basket outside each classroom for the kids that could not bear to leave a toy in the car, and the kids know that their toys will be returned at the end of the day. For our kids, we move their toys out of physical reach but in plain sight on the table... and that has helped deflect tears over having to leave something they were absorbed by and don't want to leave for a meal.
