How do you cheer up a grouchy person?
It totally depends on the person. I have no set standards but a funny joke is usually the best option. Play some funny songs (music videos if you have them), funny fart noises always seem to work. Trust me, fart noises always do the trick.
Watch a funny movie always does the job. Off the top of my head I can think of 10 movies I would watch and make other people watch to chase away the grouchiness. Or sit and talk about the good times over a beer or two. Have some good food. Maybe some harder liquour as well.
Do all the things that the person likes to do and enjoy in the fun together. That always ensures smiles.