How can I access my Google Reader entries offline? I am without a mobile internet plan on my smartphone, hence I am looking for a way to, whenever I'm on wifi, download my Google Reader entries for me to read offline on the road.

I don't mind installing a third-party app, even if it costs a few bucks.
I would start with gReader, which is a very nice unofficial Google Reader app. It supports offline syncing (and can default to only sync on wifi), and multiple other options to customize it (update intervals, number of items to sync, full page/content only/content with images).

If you like it, there is a paid version gReader Pro which removes advertisements and has some additional features.