I could name a lot more than five items but to make it more practical I am limiting myself to a pencil case/ container that contains five essential items.
Aquash brush pen: There are a few of these on the market I prefer the Pentel ones as they seem to last longer and have a valve that controls the flow of water so it's always the right amount. They come in three sizes and I usually have one of each. Brush pens remove the need for carrying water and all the mess that goes along with that. I fill the pens up regularly from a plant spray bottle that I fill with clean water. Over time the brush-pens get clogged up with slime that accumulates inside the barrel however hard you try and keep it clean. My tip for avoiding that is to add a few drops of bleach to the spray bottle; this kills any slime forming bacteria and makes the brush last a lot longer.
Neocolor II water-soluble crayons by Caran D'Ache: These are a life saver! They can be used as straight forward crayons or as water-colour blocks. I use them mainly as water-colour pans getting the colour directly from the crayon with my water brush. These crayons come in over a hundred shades and last forever, they are easy to carry and you only need a core set of about 8-10 crayons to create an infinite number of shades. I save all the little broken bits to use which makes them very economical.
Pilot G3 pens in black and brown: I discovered these pens a few years ago when I was looking for an alternative to my Rotring Rapidograph or my Koh-I-Noor technical pens. They are tough little pens that are much more robust than my others. Their tiny nib and intense ink make them perfect for outlines or complete drawings. The brush pens used with these create delicate washes that are very pleasing. The Pentel Slicci 0.25 pens are a new addition. They are even finer than the G3s but tend to dry out very quickly if I leave the cap off.
Scissors: A good quality pair with a fine tip are a must for cutting out found images (Sorry Doctor Serrano, I confess to defacing your ancient magazines during long waits!). I find my images everywhere! Sunday supplements, women's magazines, found bits of paper or text from a letter. I am not proud! Having a small discreet pair of scissors make it easier to save these juicy morsels for later use. I use a pair of good quality hairdressing scissors that I bought in a charity shop and a pair of small embroidery scissors as backup.
Glue: I use PVA and I get through gallons of the stuff! I buy it in 2.5/5 gallon containers from builders merchants. I have a small tomato ketchup squeezy bottle that I fill up for daily use. When I am working on a piece I tend to glue bits of paper over bits I don't like or any glaring mistakes. I hate it when I have to save for later because I've forgotten the glue. And no, glue sticks are horrible! So I don't use them.
So there you have it! Five items that will serve any collage/sketching urges that overcome you while away from home.
I haven't included sketch books or images because I carry these with me always as a matter of course.