It happened without warning, and goodness knows it happened decades later than it should have, but the the original Maniac Mansion is now available on Steam. Sure, you already have it as a free bonus feature within Day of the Tentacle, but don't you want the pleasure of buying it individually, especially since the last time you could do so was like forty years ago?
And anyway, it's less than five bucks. So do it. Do it now.