Macro on the cheap. Macro lenses are expensive, but you don't need to buy one.

[Major Nikon]

4. I have a couple of sets of extension tubes

You have to be careful about some of the cheap ones. For cameras that require no electrical connections, it doesn't make much difference. For the ones that do require electrical connections, I've found the cheap ones are not worth it because they tend to develop poor connections over time.

Extension tubes are great for anytime you want a lens to focus closer than it would normally. I sometimes use them on telephoto lenses when I can't get the lens to focus as closely as I'd like. They also work good with actual macro lenses to increase magnification.

Diopter lenses also work well for macro on the cheap. They screw on the end of the lens like a filter and provide additional magnification at the cost of varying degrees of additional optical aberrations.