Note: I imagine Harry did this a lot.
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When the Dursley's all went out to dinner or some other thing and left Harry at the house and not locked in his cupboard or at Miss Figg's house, Harry liked to take his chance to spite his family. He didn't blow up the house, like Uncle Vernon feared, or tore up the furniture like Aunt Petunia thought he did in his spare time, but he just did some of the things he wasn't ever allowed to do.
As soon as the lights of Uncle Vernon's car was gone, turned at the corner of Magnolia Crescent and Privet Drive, Harry would zoom up the stairs and fling himself on his Aunt and Uncle's bed. (He'd learned the hard way that Dudley liked to keep sharp, pointy things hidden in the sheets and crumbs littered it.)
Their bed was soft and bouncy and Harry liked to just lay there in the warm blankets for a while. He wouldn't let himself stay very long just in case he might fall asleep and then get into some real trouble, but he stayed long enough to enjoy his time.
After remaking the bed as best he could to look as it was before he'd disturbed it, Harry liked to venture around in Dudley's room, salvaging any books he could find hidden in the depths of his dirty clothes mountain or the unexplored jungle of his wardrobe. Any books he'd find he would hide under his cot in his cupboard and then head to the kitchen.
Rummaging all the way to the very back, Harry would take anything that looked like Aunt Petunia may throw out soon. She never missed them anyways. Harry would munch away at a bruised apple or scoff down some iffy leftovers that Uncle Vernon never took for lunch at work and then he would rearrange the shelves back to their original selves and the hideaway back in his cupboard before his relatives could get home.