As noted you can use any items that can be exchanged.

For my example I found backpacks useful. As noted stackable items that have 'stacking space' available will just stack and not exchange.

1. Place one backpack on your player's back
2. Have the other on your person (holding, in inventory, doesn't matter)
3. Walk into a large bush preferably one that is taller than the player. (see picture below)
4. Drop the backpack that is not on your person.
5. Drag the dropped backpack onto your character to exchange it with the one you're equipping.
6. The items will swap and the bag you had equipped is now hovering over your head at the top of the bush.

-Only way to get your item is to cut the tree/bush down.
-I had this first happen while swapping between two vests but I've also seen it with pants and shirts.
-With a evergreen tree it often gets stuck 2-3 branches up (not sure about other trees)

Note this can happen with any items in the game as long as they're not of the same "stackable" type and still have "room" to stack (otherwise they'll just stack rather than exchange place).

They also won't happen if the item is in the players hands and or can be moved to another point on the character (for example if you you have a long stick in your hands and space for it to move the long stick will move to your back slot. However if you do the same thing and don't have space the stick will get stuck... In any case the backpack is one of the easier ways to get this to happen.

This is minor but it can be very very annoying if you don't have an axe and it happens with a tree...

This screenshot shows what it looks like after you cut the bush down if you exchange rags with a long stick.