Ask HN: I've got 3 months of long weekends. What should I build?

Hey, My wife and kids are taking an extended trip overseas. I have organised to take 1 day a week of work so that I can spend some time on personal projects. My current idea is to try building 3 small projects, spending a month on each and blogging the results. Each project should:

* Be capable of producing a small, continuous income stream, with only minimal extra investment once it is complete.

* Be small enough to complete in a month of long weekends.

* Have some easily measured metric, that I can track, and use to iterate and improve my work (pageviews, downloads, purchases, etc)

* Require only a small upfront investment in cash.

As for my skills I have 10 years experience in software development, can design, write and take photos. The current projects I'm considering are:

1. 1 month of stock photography, uploaded to a site like http://www.istockphoto.com/

2. Creating themes for magento, or some other e-commerce or CMS platform

3. I have an existing website that sells products over the net ( http://www.grafting-tool.com/ ), however it gets very few customers. Spend a month trying to increase the pageviews and page rank.

4. Android application to allow people to sort through their photo collections on their phone.

5. A solution to link Google analytics conversion tracking with paypal buy now buttons. I spent ages working on this on my own website and there is no easy off the shelf solution.

What are peoples suggestions and experiences with side projects? Is there anything I should be looking at? Or avoiding?

I hope to share my experiences with HN, so in just over a month, I will have my first success, or failure to report.

Edit: formatting.