The biggest problem with a long-time bondage session is dealing with the boredom. So the best way to deal with this, is not being tied up for the whole duration but put it into chunks. Of course you also have to eat, drink and sleep too, so doing it 24 hours is already going to be really hard if you take away food and drink.
It would help if you can add if its solely selfbondage that peaks your interest, or if you are also allowing yourself to suffer a little bit. This last part will make things much better as you open up for far more possibilities.
If this is allowed, then I can highly recommend nettleplay. It is going to be very scary before you start, but once the first nettles hit, you will be craving for more. Keep in mind though, that nettles dry out quickly. You can't get them to do a session the next day with them because by then, they'll have dried out and died.
If outdoors also means do a session somewhere outdoor in nature with nettles, then it will be very nice to put lots of nettles in your undies once you leave the forest area with an extra bunch in a bag to continue the session at home.
If you never even has been stung by nettles, test out a small sting on your forearm, just to ensure that you aren't allergic for it. If you are, it will get like 10 times larger than normal with big swelling and you will feel much more sick. Normally it will itch and tingle a lot like you feel electricity. If its far more than that, then abort.
Also, google for some nettle sting images so you have a vague idea how small the blisters are supposed to be and compare.