What's a good piano piece to work on if I just finished up Clementi - Sonatina 1?

I recommend "The Entertainer" by Scott Joplin. It's a different style, but about the same level of difficulty. Varying styles can broaden your horizons and help you to sight read better. Some others you might wish to try, are Bach's Sonata in D, and perhaps some Scarlatti pieces.

Clmenti Sonatina No. 1 is around Grade 3 level at ABRSM scale. http://www.classicalmusicdb.com/musics/view/1259

If you are getting fairly comfortable with it, you can start looking into ABRSM Grade 4 level pieces, or check out RCM Grade 5 pieces which are about equivalent in terms of level of technique. Most of Mozart's Sonata are much higher grade (around ABRSM Grade 7 or RCM Grade 9) so it might be a huge jump.

You can check Classical Music DB for the pieces in each level, and pick something you like.

http://www.classicalmusicdb.com/levels

Since both Mozart and Clementi are from classical period, I'm guessing you like musics in that style, if so you can check out list B from the RCM levels, which are musics composed by classical period musicians.