Midway through Toy Story there was a scene where Sid is called off for his poptarts, Then we switch off to see Woody’s head burning. Then the Hallway scene, then the “I will go sailing no more” scene, then the Mrs. Nesbit scene. Then the whole Woody talking to the other toys across the alley scene, then Buzz getting his arm fixed, then finally Sid finally comes back. What took him so long? Must’ve been a lot of poptarts.
He was waiting at the mailbox for “THE BIG ONE”
Hah, That could be it.
Maybe he was blowing up Hannah’s dolls.
Maybe somebody put laxative into his poptarts. Let’s face it, the kid deserves all the crap coming his way (no pun intended).
I also wonder why it took Hannah so long to realise that wasn’t her mother.
She was probably looking around the house for her.
Maybe after he’d finished his poptarts, he was destroying more toys in his back garden, or was waiting for “The Big One” like YUS suggested.
Who knows, but to me, it seems that Sid has a VERY short attention span. That’s why he stops looking for Woody, and how his mutant toys survive by hiding. He just forgets about it.
Had there been more explosions, I would’ve assumed he would’ve came back to his room to get the matches.
I rather like the laxative pop tarts idea. 
No wonder he’s so skinny! LOL! -Omar
Enjoying his pop tarts, for all 15 minutes of it. But realisticly, something may of cuaght his attention and drawn him in for 5-10 minutes, seeing as things like that happen.
While they didn’t talk exactly about this, John Lasseter and Andrew Stanton mentioned a similar thing they were always commented on at that point in the film during their DVD commentary:
“It’s like 5 or 6 o’clock! How did he get this box so late from the postal service? Who delivers packages that late?!”
“I don’t think people will notice…”
“Who cares?!”
“He just got home.”
That last comment seems to suggest that Sid was at school or something all that time after having his breakfast. Andrew even referred to the type of people asking those questions as “weird logic police”.
Flik-E: But packages do get delivered around 6:00 here. 
Same here, nothing unusual about the mail coming that late. And there’s no school since it’s summer. The fact that he got distracted isn’t that hard to believe.
That’s right, I forgot that he was kicked out of summer camp.
There is a such thing as summer school. It’s usally like 3 hours shorter so mabey he went to a friends.
There’s no point to being in summer school, since he’d been expected to be at summer camp during that time period, before being kicked out.
Maybe his poptarts burned and there weren’t anymore, so he had to go to the store and buy some.
Maybe he was sitting around a campfire with Spike making delicious hot schmoes. 
Nah, he would have been eating Schmoe flavored Pop Tarts.