It actually depends on the kind of sound experience you want to get. When I am outdoors, I always carry a bluetooth speaker and the only problem is that its battery doesn't last for long so I have to keep recharging it.
Are you looking for something that is wireless and Bluetooth compatible? I found speakers that are by Bose that blend into your landscape that are Bluetooth and Wi-Fi enabled. They are pretty pricy, but I bet they sound amazing.