Violin

Mesh fabric fastened flush to the bow, creating a gritty, broken sound (used for a variety of "articulations" such as tremolo and harmonics)
Felt fabric fastened near the bridge to create a softened/muted sound (most notably for "Soft Violin" but other sounds as well)
Attaching metal and other materials to the strings to achieve a different sound ("paperclip violin")
and more....

Electric Guitar/Acoustic Guitar

Home-made soft guitar picks or soft "found" material for the normal picked sounds
Felt fabric and other fabrics used to mute guitar harmonics
A technique used for the "moving" plucked and harmonic sounds was to move where I was plucking the string in order to create a different tone across the time of the sample
Isolating and amplifying reverb tails
Notes generated only from vibrating the strings with the left hand and fingers, no plucking ("ghost guitar")
Notes generated only from a hammeron and then turning the volume knob up ("volumation")
and more...

Bass Guitar

Normal bowed
Mesh fabric bowed (similar to the violin, but not as "tonal")
Bowed in a "sweet spot" near the bridge in order to create a gritty, harmonic-like, dark sound (these are the "Gritty Harmonic" sounds)

Piano

Muted pedal sustain (pretty basic)
Muted on and off with fabric (at the strings) while rapidly pressing the keys
Muted pedal sustain pressing keys rapidly with a varying degree of dynamics
Live performance of rapidly playing different intervals (octaves and whole tones, "octave etude" and "debussy's dream" respectively)
And more...

Mandolin

Bowed
Multiple strings tuned to same note and plucked together ("mandolin unison plucked")
Basic pluck with a home-made "cardboard" pick
Fidget spinner
And more...

Ukulele

"flamenco" style plucked --using 4 fingers and rapidly plucking the same string
"brushing" notes with felt fabric