(idiomatic) to stop at nothing

(informal) to play the field; to have several romantic relations at the same time; to take every chance that presents itself

it can't hurt to ask

(colloquial) Said of a failed (but costly) project.

(idiomatic) to be up to no good

(there is) nothing new under the sun

Alternative form of nichts Neues unter der Sonne

"He who wants to have everything, gets nothing in the end"; One cannot like everything and give equally to everyone, therefore must take sides and choose.

"where nothing is left, even the ruler has lost his powers"; only talent and beauty have real value

Things never turn out quite as bad as they seem at first (or as pessimists/alarmists make them seem).