Portable NoSQL (SQLite alternative)

I'm developing a project that is currently using SQLite. The benefit is that its highly portable, no configuration on the user's end (simply download the app and run), etc. But I'd really like to use a NoSQL DB for this project. Started looking into things like MongoDB, but figured I'd ask here.

What is the best NoSQL DB alternative to something like SQLite that is portable? Not sure if it matters, but I'm using Ruby for the project.