I'm new to AppEngine and I'm familiar with Django, so that would have been my first choice - but it seems like Flask, Werzkeug, Webapp2 are more popular - just curious why, or how good/bad Django is on it.
https://github.com/django-nonrel/djangoappengine
https://github.com/potatolondon/djangae
There's compromises in using the datastore - you can't do a lot of things that are dead simple with SQL, but in return you get massive scaleability. And the App Engine SDK itself can be a pain to work with, but there are also amazing features like task queues for background processing and Google Cloud Storage integration.
Other than Django I've used Bottle for very small App Engine projects. Works great, it's faster than Django, I'd still prefer Django for a typical complex web app where you need all the features.
Disclaimer: I work for Potato, which sponsors djangae development.