When sending your Pi away on outdoor missions, the two main concerns that need to be addressed are the supply of power and protection against moisture. A lithium polymer battery pack is a good choice for powering the Pi off-road. They are usually marketed as portable smartphone chargers, but as long as yours operates at 5V, and provides one or more USB ports with around 1000mA of output (or 2000mA for the Pi 3), it should keep your Pi happy and running, usually for five to ten hours. If you need a USB hub for your peripherals, make sure it can be powered by one of the USB ports on the battery pack, and not from one on the Pi.
The iMuto X4 type shown below (usually used as a phone charger) is one I use when I make my Pi portable and will power a Pi 3 for around 12 hours, depending on what else I have plugged in. It also has a separate output that can be used to power a separate USB hub if required: