get a piece of card and cut and square out of it
then find a large picture and lay it flat - throw the piece of card onto the picture - then wherever it lands - that is your modern art piece!
it will likely be an unidentifiable collection of textures, colours, shapes etc
but it could just be a close-up of something - depends on the painting and the size of it
keep throwing until you find one you like.
another thing is a still life.
clear a table, then go round your house and collect a random selection of objects - anything - doesn't have to be nice things - then arrange them on the table - not individually but piled and styled - then paint what you see
you could also add a light source to one side of the collection to add some strong shadows and highlights.