Mini Piano is a very fast and beautiful piano app that gives you access to 5 full octaves, a sampled piano and 128 different midi sounds. It supports multitouch, if your device has the hardware for it. Some devices only support 2 fingers, but the app is capable of handling all your fingers. The app is optimized for tablets and phones.
You can use the zoom controls on the keyboard to adapt the keys to your device. Maybe you need more keys on the screen or you need less keys to make it playable. It's all possible.
The app is ad-supported. You can change the volume with your phones volume keys.