Macro filters. I have never tried these, and can therefore not give a first hand review. But, the method is that you can attach a filter in front of your lens, and achieve some result. They work like magnifying glasses.
I have anyway chosen to add them to this list of ways to do macro photography, because they are available, and because they are fairly cheap.
Macro filters are graded in diopter, where +1 being less and +10 being more (closer focus). You can also stack the filters. When stacking, you should place the filter, with the highest number, closest to the lens.
With macro filters it can be harder to focus.