Very simple. Whole movies take 15-30 minutes tops.
Andy comes home from college in the summer, and in order to make a bit of spending money before he gets a part-time job, takes over watching Bonnie. She shows him how clean and neat she kept his toys, and how even added all the new characters from 3. Her and Andy play with them, as a young kid and babysitter do.
We see Andy play with Woody again, and even if Andy can't see it, Woody starts to tear up. After all this time, even for a little bit, Andy's playing with him again. For just an afternoon, it's back to normal. Andy never really forget his old cowboy pal, heck, he could probably remember the names of the old gang still.
A few months go by, then a few years, and the movie shifts to a yard sale out at Bonnie's house. She's going to college herself, and she's getting ready to sell some stuff to buy books.
A car pulls up, and a man, a woman, and a little boy walk out. As the man and Bonnie talk, the boy walks over to a big box sitting in the corner, and opens it. Inside is Woody, Buzz, the Potatoheads, everyone. Bonnie offers the little boy the box for free, despite his father's objections.
"Aw, come on, Andy." Bonnie laughs. "They are your toys, remember?"
Andy looks at his son, seeing him play with Buzz and Woody, laughing as he pulls Woody's string or turns on Buzz's jetpack. He smiles nostalgically, and agrees. They load the toys up, and drive back to their house down the street.
Andy's son carries the toys up to his room, sits down and starts to play with them. the sound of laughter and Woody's voice echo out as the scene draws to a close.