I am actually underwieght and can't seem to put any on :/
Weight training.
I once put on over two stone in a matter of weeks, when I was underweight. I wasn't even organised and dedicated - I just got a bench and some weights and used them on an unscheduled basis. Having a shower? Do a few minutes of various barbell exercises first - you can get sweaty, as you're having a shower anyway. Making a cup of tea? Do some dumbell exercises while the kettle boils. Downloading a fairly large file? More dumbell exercises while it's coming down. Or while you're watching TV. You don't need to move much with a dumbell. I'd also do maybe 15 minutes a day, lifting until I couldn't lift it any more (which isn't really a good idea with bench pressing on your own). As another plus, that would get me high. Off my head, laughing at the ceiling. Your brain makes excellent drugs for itself.
Two months of that and I couldn't get into my shirts. The arms on my T-shirts cut into my much larger biceps and the collars on proper shirts strangled me. I had to buy extra weights twice - weights I could just about bench when I started felt like an empty bar after a while.
Your food bill will go up, though, because you eat a lot more. Muscle is resource-intensive.