As per the question title, I'm interested in starting up a new side project and I'm currently looking at doing a bit of mobile development as said project. I'm actually in a bit of a good position right now as I have a blank slate in front of me (i.e. not locked into a phone already) but on the same token, I also don't want to invest too much money into hardware and the like since this will likely not be a money making venture.

As such, which platform provides the best flexibility in terms of experimentation and learning opportunities? Likewise, what are some useful resources to assist in the process?