I just found the ultimate power pack for when going outdoors and working portable. It is called the Goal Zero Sherpa 50, weights only 544 grams and has 5V USB (to recharge phones/tablets), 19V @ 5A (to recharge laptops) as well as 12V @ 8A (more than enough for any QRP transceiver) outputs.
What I really like about it is the fact that it includes all the electronics necessary to manage charging, balancing and protecting the battery plus the converters that transform a wide range of input (14 to 25V) and internal battery voltages to 19V, 12V and 5V for the outputs.
The energy is stored internally in 3 Li-NMC cells for a total of 5200mAh @ 11V. I haven't heard of Li-NMC cells before so I had to dig around a little: apparently it's a type of cell with a very high energy density per weight, putting even the regular "lightweight" cells to shame. While the typical sealed-acid battery has a density of around 40Wh/kg, the latest-technology NiMh (such as the Sanyo Eneloop that I use in my Yaesu FT-817) gets close to 90Wh/kg and the widely popular LiFePO4 touch 120Wh/Kg, Li-NMC cells offer an energy density of 200Wh/Kg !
An that is not all, the manufacturer offers alot of accesories you can use with it: solar panels to recharge it in the field, an inverter you can attach to get mains voltage output, different adapters/cables etc. If you're not satisfied with the 5200mAh capacity, there's also a 8800 mAh version called the Sherpa 100.
The only downside to it is the price, but if you really use all the features it has you'll probably get your money's worth.