Musical U: This website is headed by a great music educator named Christopher Sutton. It's an outstanding resource for all things related to Music Theory, including ear training, scales, chord building, the Circle of Fifths, and more! It's subscription based, but there's tons of free resources!
Ukulele Tricks: Brett McQueen has tons of free and paid resources on his website, for beginners and intermediate players alike. He's got over 66,000 subscribers to his YouTube channel, so he must be doing something right! We're so lucky to be musicians in an age with so many resources available! :-)
Recommended Ukulele C-Tuning Resources