|Cheap plastic air mattress|
As we only travel with carry on baggage it has come into use when we have had long delays at airports. You can usually find a quiet corner and set up your bed using sarongs as sheets.
If the pillows in the hostel are not so good the mattress can be put across the bed to create pillows for a double bed, if the mattress is poor you can put this on top or underneath for added comfort.
On the beach it makes a comfortable bed to lie on and is great fun in the water. In Florida we used ours to river raft in the hot springs saving the cost of hiring the tubes.
We find the extra weight is negligible compared to the many uses.