The cajon is probably one of the most useful, easiest drums for any scenario these days, but especially outdoor accoustic jams, be it on the beach after surfing, at a garden party with friends, when out camping, etc. One very useful scenario  for the cajon (and one that I am very familiar with)  is also busking or street performing. If your cajon is of a decent quality, it is definitely loud enough to play accoustically even with other instruments miked up at a rather high volume, and it sounds great! And the fact that the cajon isn’t entirely commercialised yet just adds weight to the already-amazed reaction that ppl has towards this simple box and its ragingly powerful sound! It is alo a very easy instrument to carry, isn’t very fragile so you can take it outside and be a bit rough, plus, it would mean you also always have a chair with you :-).
 
Here’s a video of Heidi Joubert busking with 2 other cajon players (3 cajones) and 2 guitarists in central London this week. Note that the 2 guitars are amplified to a rather high volume, but none of the cajones are miked up. If you are having dificulty with getting enough volume playing accoustically, if there is a wall or solid panel around, simply position yourself in a position that faces the sound hole of your cajon towards the wall,  that will amplify your sound as it will bounce off of the wall and be projected forward. (Thats why I love the sound hole in the back of my cajon for busking, it would really suck to perform looking at your audience from the side unless this is something you would want of course)