Is a Ukulele.
Nothing (I have found) sounds so light and friendly and inspires such a happy mood.
We were lucky enough to have a friend sing at our wedding and among the song suggestions we gave her was 'Somewhere over the Rainbow' by Israel Kamakawiwo'ole. She came back saying she was unable to find a backing track for it,but had almost mastered it on the ukulele so would give it a go live if we wanted.
I was incredibly excited about this and she was amazing. It made me cry!
This inspired me to have a go at learning the ukulele myself and just after Christmas I bought myself a cheap one and some books.
While the instrument may be cheap (I got it from Amazon for less than £20). It has a lovely sound, I don't feel that I need to immediately invest in a high quality instrument, although I may at some point. At the moment my lovely (yellow) ukulele is doing me proud.