1. Sid's carpet

2. The number 237

3. Mickey Mouse

4. The gas station

5. Lightning McQueen t-shirt

6. A Bug's Life toy

7. And John Lasseter

I bet you're going to go re-watch Toy Story after #4.
In the first Toy Story film, Woody and Buzz are escaping Sid's room and the carpet happens to be identical to the carpet from The Overlook Hotel in The Shining.
Another reference from The Shining — the number 237 appears numerous times throughout the Toy Story franchise. 237 is the room number of the infamous haunted room in The Shining.
The watch on the wall has Mickey Mouse on it in the first Toy Story movie, serving as one of many hidden Mickeys throughout the franchise.
The gas station featured in Toy Story is also the name of a racing team in Cars, which was released several years after.
In Toy Story 3, one of the daycare kids is seen wearing a Lightening McQueen t-shirt with his number 95 on it.
The bugs from A Bug's Life are shown behind Buzz in Toy Story 2.
A magazine in Toy Story shows the date January 12, 1957 in the bottom left corner. This is also the date that one of the directors of Toy Story, John Lasseter, was born.