。
He took advantage of the good weather and pottered around in the garden.
Literal
He [topic-は] good-weather [object-を] use-and garden-tinkering [object-を] did.
好天 ('good weather') is the formal/literary alternative to 良い天気. 利用する ('to use, take advantage of, make use of') has a slightly opportunistic feel — distinct from 使う ('use'), it implies leveraging a circumstance. 庭弄り ('gardening, garden puttering') uses 弄る ('to fiddle with, tinker, mess with'), giving a casual, hobbyist flavor — quite different from the more dignified 庭仕事 ('garden work') or 園芸 ('horticulture'). Together, 利用して庭弄り paints a picture of seizing the day for unhurried garden tinkering.