I would develop a SaaS app idea over your time away from your day job.
Choosing a language or skill that's going to be the next big thing is tough.
Ruby on Rails would be a good choice to use to develop your app and Rails developers are in demand if it works out you go to work for someone as a developer in the future or freelance on projects.
Since you're around 40 I would work toward having your own business rather than programming for someone else with a new skill.
Comments
I would develop a SaaS app idea over your time away from your day job.
Choosing a language or skill that's going to be the next big thing is tough.
Ruby on Rails would be a good choice to use to develop your app and Rails developers are in demand if it works out you go to work for someone as a developer in the future or freelance on projects.
Since you're around 40 I would work toward having your own business rather than programming for someone else with a new skill.
Check out http://www.startupsfortherestofus.com/.
They have a lot of good information on their podcast.