My son suddenly thought he would like to start puttering around in the garden.
Then again in the afternoon, out of nowhere, he decided to make a fruit salad for us.
ーMy son suddenly thought he would like to start puttering around in the garden.
「息子は急に思い立って庭いじりを始めた。」
suddenly thought を使って「急に思い立った」を表現できます。
=He suddenly had the idea of puttering around in the garden.
suddenly had the idea of ... で「急に思い立った」とも言えます。
ーThen again in the afternoon, out of nowhere, he decided to make a fruit salad for us.
「そして午後にまた、急に思い立って家族のためにフルーツポンチを作ってくれた。」
out of nowhere「出し抜けに・唐突に」を使っても「急に思い立って」を表現できると思います。
ご参考まで!