Hi Zoe. Yes, the ukulele is very easy to learn and a lot of fun. Its chief use is to play chords while singing rather than single-note lines like a viola. I recommend the concert size ukulele for most beginners. It's a little larger than the soprano size (it's kind of like the viola of the ukulele family) and easier for most people to fit their fingers on. Unlike the piano and the viola, the ukulele actually IS an instrument that you can learn to play well using free videos. With your experience you'll have no trouble at all.
Rather than reading notes like with the viola, the ukulele generally follows the chord symbols. You've probably seen piano music that has letters and numbers above the music like C, F, D7, Am. Those aren't for the piano but are the symbols used for guitars and ukuleles. As a ukulele player you learn what those symbols mean. Each symbol represents a chord. Each chord is made by placing the fingers in certain spots on the fingerboard of the ukulele. It's easy once you get used to it.
Believe me, once you get the ukulele in tune you will be playing your first song in five minutes!