Make a simple sugar syrup (water and sugar) then depending on the flavor of the cake add some flavor (a small amount of mint works great for plain sponges) - put that in a a spray mist bottle and you can moisten a cake.
If its a fruit based cake then alcohol

- brandy, rum, cointreau - At a push grate some lemon zest and add that to vodka and use that.
After you have moistened the cake make a butter icing or glaze etc.