no-brainer 
noun informal
Something that requires or involves little or no mental effort: the enormous popularity of his TV show makes the book a no-brainer for him...
- The exams now consist disproportionately of no-brainers like: ‘When you were in school, were you a member of a sports team?’
- Counting these no-brainers as evidence of genius is absurd.
- This is a no-brainer, but one of the biggest mistakes hikers make is not realizing the total length of the hikes they are on.
- be tuned in
- between
- between a rock and a hard place
- between jobs
- between ourselves (or you and me)
- between the devil and the deep blue sea
- between you and me and the bedpost (or the gatepost or the wall)
- between you and me and the gatepost
- between you and me and the wall
- be twice the man/woman (that someone is)
- betwixt
- betwixt and between
- be two a penny
- be two (or ten) a penny
- beun
- be unable to believe one's eyes (or ears)
- be unable to hear oneself think
- be unable to (or be hardly able to) believe one's luck
- be unable to punch one's way out of a paper bag
- be unable to see the wood (or the forest) for the trees
- be under no illusion (or illusions)
- be under the illusion that
- be up shit creek
- be up shit creek (also be up shit creek without a paddle)
- be up the creek