Google has released a new app that is designed for simpler forms of scientific experiments.
The app also supports any external sensors.
The ideas can be collected in projects and you can make notes and gather data for multiple experiments and see how the results differ between the trials.
Those suggestions on experiments to perform'll find a collection here.
You can use the phone's own sensors or connect external sensors to carry out experiments on your environment.
Organize ideas in various projects, formulate hypotheses, write notes, and collect data in several test runs.