Sing! It's hard to be anything but happy when we sing. Try to keep from judging your voice. Don't compare it to other singers. Just sing.
Another suggestion is to do a good deed for someone else. This brings a feeling of joy that will last.
Walking, dancing, swimming, biking are all good happiness boosters as well as being good for the body and mind.
Laugh! Find something to laugh at. You tube offers all kinds of hilarious videos.
When we are "down" and feel blue - it's a state of mind. Change your state by interrupting your thoughts.
I hope these tips will be of help. Thanks for a very good question.