A lamp?
Upsidasium ?
I was thinking helium too, but when I really thought about it, a full helium tank is heaver than an empty one. So I guess you'd need to preface it with what was is already in the barrel and is it sealed? A helium tank(barrel) I guess would be lighter than a air tank but I really don't know, because you'd also have to qualify it with the pressure of each. If they were filled to the same pressure, I'd think you'd need a lot more helium because of it's small size, compared with relatively larger atoms that make up your standard air mix, but then again you said put it and didn't mention taking out. Putting helium in without removing anything would only make it heavier. I guess it all boils down to a question of density, so I guess I'll stick with my original answer, A lamp.