When Petunia woke up that morning and found her nephew and the letter she made a vow to herself to stamp the magic out of him. Magic would not steal her nephew like it had her sister. As she pondered on the best method to "stamp" the magic out of him - she remembered how Lily's husband was a prankster in the wizard world. She decided she would make her nephew a prankster like his father. Then she could make him believe magic was just a prank.
8years later
Petunia constantly planned pranks on her boys, she considered Harry her boy and she loved him dearly. All of her pranks helped her to lose the grief she had for her sister's death. Whenever she began to feel anger that her sister had abandoned her - she would put hte energy into planning a prank. Which was part of the reason her nephew now looked like a mummy. She had decided to toliet paper... him. Ther other reason was thta she had decided seh really loved pranking, even if her pranks weren't necessarily good. But this is how her family bonded.
Harry loved his aunt and he knew that praning was his calling in life. He liked pranking - and being pranked, because he figured he could learn from other people's pranks. He decided that a good counter prank for his aunt would be to make their yard a museum. Many people thought Harry wasn't very smart because he spent so much time pranking, this however was false because his pranks all required research. This is why he went to the library. To research things that go in a museum and how he might be able to make a believable replica. He made a mummy, ancient scrolls with ancient writings he found while researching. He made ancient records that would resemble stone plates. He got cardboard and cut it into reasonable rock looking peices and then covered them in tin foil with the dull side up. Then he used a pencil to push in the cardboard and tinfoil. It resemble something you might see. He made random paintings, he made statues, dinosaur bones. He spent the whole day working on it. After making all of his objects he set it up during the night in a tent. He put up signs about a traveling museaum. Right next to the exit he put a sign that said you've just been pranked.
When Petunia looked outside the next day she had a little laugh. She didn't know if it was her son or her nephew , but there was a huge line outside her house waiting to get into a tent. She went to have a look. The museum did look genuine, but she wasn't fooled. She knew it was a prank.