I read a book in the late 90s or very early 00s, and I know it had a green cover, crazy green colour with a lot of activity on it too. It was about some kids who discovered this other parallel town or city or something. They were just average kids. I'm not sure on appearance as it's been a very long time since I read it, and I'm not sure if they were young teens or slightly younger. I'd guess between 11-13 years old, but they were just normal kids. I think there was a main male lead and a female who was a kind of sidekick character who ran along with him. I can't remember how they traveled to the town, possibly by touching an object.
The main thing I remember is that when they traveled to this place they still remained in their original place too and would just be like zoned out. And I remember one scene where one of the kids is riding in a car and wanted to do a quick travel there so he does. Also time worked differently, so a second in their actual world was like a day in this discovered world.
I loved it when I was a kid but cannot for the life of me find out what it was. I was around 10ish years old when I read this book.
I hope someone can help! Thanks in advance :)