Collocations for mattress - noun

double, king-size, queen-size (AmE), single
comfortable, cushy (AmE)
firm, hard
soft, springy
lumpy
an old bed with a lumpy mattress

thin
air, blow-up, inflatable
foam
cot (BrE), crib (AmE)

on a/the mattress
The children slept on mattresses on the floor.

under a/the mattress
My grandmother keeps her money under the mattress.