RFO BASIC! for Android. Fun system to play interactively with Android stuff - and especially to do that on the go. Basically a souped-up Basic with support for various Android sensors and access to all kinds of system services (e.g. SQLite), so you can really do a lot with it. Basic programs can even be converted into APKs that can be distributed as standalone apps. This brings back the feeling of programmable calculators from the 80s - amateur programs, but self-made hacks and tools.