[melle]

Depending on the type of UI, you could also give Kivy a try. https://kivy.org/

It's python based, and really easy to get going. It does have it's quirks, but overall I found it a joy to use.

[BucketSort]

I've had some hard times porting Kivy to the Raspberry PI 3 on Raspbian. It is a decent toolkit, but I'm not so sure on the ease of cross-platform.