which are, in increasing order, the easier music instruments to play?

My vote is cowbell for easiest and theremin for being pretty tough,
though really any instrument you practice is easy to play and any one
you haven't is gonna be tough.

Any instrument is going to take some practice to play *well* but they
are large differences in how easy it is to play an instrument *at
all*.  Pretty much anybody can get sound from percussion instruments,
while I understand that it usually takes a lot of practice to get any
sound at all from something like a french horn.

I think that apart from some general trends, arranging individual
instruments in order would be pretty subjective.

When my child was entering band the instruments were classified by the
band director as to difficulty.  This rating system is also used by
large instrument rental agencies such as Breber Music.  In order they
are ranked as:
Percussion - single skin - cowbell, tympony, etc.
Percussion - multiple skin - bells, xylophone, drum set, etc.
Strings - 
Piano - 
Horns - Trumpet, saxophone, tuba, etc. with French Horn being rated
the hardest to learn to play

Depending on the weather, wind chimes can be quite easy to play

For wind instruments or piano, your fingers go up or down - not too
much dexterity involved.

For a stringed instrument one hand is doing something independent of
the other, like rubbing your stomach and patting your head at the same
time.
The hardest stringed instrument may be the Sitar with 18 strings.
But the hardest instrument may be the one you love the least.