Close up filters are probably the cheapest option. Optically not the best, but allow full use of cam functions, and no loss of light.
A lens reversing ring will allow some proper close up stuff, but then you've got no aperture control/exposure info of the lens, and your delicate rear element is exposed (!). Not all dijical cams will support this method; lower-end Nikon DSLRs might not allow this.
Standard lens and extension tubes will facilitate really close up photography, although again there is the lens/cam interface issue. Loses a fair bit of light.
Close-up bellows attachments enable great control of level of magnification, but are pretty expensive and specialised. Again, the lens/cam interface problem strikes again.
Dedicated Macro lens is the most convenient solution, and will allow full control of aperture and exposure info, but cost loads. A decent s/h one can be had for £2-300 or so possibly. No loss of light.
Combinations of these various solutions can bring ridiculous levels of magnification, but proper macrography is a specialist art.
Close-up filters are probably your best bet for what you describe. Be careful with lighting; such filters are inferior to expensive lens optics, and more prone to flare/abberation. Some may produce slight unwelcome distortion; expensive ones are less prone to these issues.
HTH.