My son got his first LP fire engine and farm blocks when he was 6 months old. Since then, we have added A. ambulance, rescue car, garage and tow truck, garbage truck, police car and police station/post office, postal truck, construction crane with some dump trucks, and A. airport. He recently turned 5. These are some of his favorite toys.
He currently is playing with the LP airport with his Duplo bus and his Spiderman, Batman, and Superman action figures. They are trying to rescue his toy story action figures from the Evil Doctor Porkchop who has turned into a ghost (He used a tissue as a ghost costume.) All that and there is a Formula 1 car race taking place with the LP people as spectators on the ramp of the airport which was turned into a bus station. Now the alien from toy story has captured everyone and is taking them away to his jack-o-lantern shaped planet via his little purple basket that he turned into a spaceship.
The LP sets hold up well, and are great for getting them to use their imaginations. Encourage your kids to combine them with their other toys. Let them decide when they are done playing with them. I think it is so sad when I see little kids age five (some of my son's Sunday school class members) only playing video games and collecting cards because that's what their older siblings do. My friend's son who is 8 doesn't seem to mind playing with my son's LP sets when they have visited us, even though he always brings some video game thing to play while here.
We also have the LP nativity scene that we only have out from November 1 until around December 29. We are living in Japan now. Christmas decorations go up November 1 here until December 24. If they are up, then, I get the set out so he can stay focussed on the real meaning of Christmas. We keep them out until we take our decorations down.