The next step after mixing and matching different layers is making some of your own.

Why make your own map tiles?

Creating a new base map design can be a daunting task.  Who makes their own custom map tiles, and why?

The neighborhood news company EveryBlock decided to use their own map tiles to have a unique appearance, one which helped neighborhoods stand out instead of highways.

Jeffrey Warren wanted to make role-playing game maps of cities, so he used World of Warcraft graphics to make a map of London.

ESRI's Local Government maps include a Mobile Night layer to make it easier for maintenance workers to see map features on their computers in the field (third image on this post)

Getting started with your own OpenStreetMap tiles

CloudMade, which also produces Leaflet, helps you design your own OpenStreetMap tiles completely online with their Style Editor.

The main OpenStreetMap tiles are created using the open-source system Mapnik.

TileMill is a desktop application based on Mapnik, but has additional features to help you import, redesign, and export map tiles.

Google Maps Tiles

Google Maps has a style editor. I'm not sure if you can use those tiles in Leaflet.