Which controls to use in adventure game scenario

I make a mini adventure game for Android 5.0+ (code available on http://github.com/montao/adventure) where the player can move between places and NPC can enter and exit the places and perform random actions and random events can occur. The game engine works suprisingly well and is written for this purpose. A screenshot looks like the following.

I found that there is no more room for more buttons and I might want to add another button, is there any good way to solve this problem given the limited screen size of the view?