It's most relaxing when it's just the two of us as a couple.

Literal

Married-couple just-us [subject-が] carefree [copula-で] good.

水入らず (みずいらず) means 'just among ourselves / no outsiders' — 夫婦水入らず is a set phrase for a couple spending time alone together without children or guests. 気楽でいい = 'comfortable and nice.'