BartVanDerWolf, that's quite some spider! Excellent catch.
Edmund, what magnification do you need? Extension tubes are practical and don't have optical degradation because there is No Glass. BTW, off-brand air is of equal optical quality to Canon air. You can try out tubes using an el-cheapo set for $30.00 off flea-bay; the mechanical quality may not be great. This may not matter much if you are applying a lightweight normal lens to the tube. If you plan to stack tubes and teleconverters (see below) with a heavier long telephoto lens, I would suggest that you buy the Kenko set, $200.00. But that's just me, I feel better if my heavy glass is attached to a beefy tube set.
If you want to calculate the estimated magnification available with extension tubes, there is a calculator at
www.cambridgeincolour.com in the macro tutorial section. You just need the lens focal length and its original maximum magnification (look in the lens specifications online, I just go to the manufacturer or to B and H), and insert the length extension tube you want, and the page will do the calculations.
One thing to remember is that you have to add the extension tube length to the lens length for total new length, and make sure that there will be some distance between your subject and your front element. Reversed-normal-lens macro is fun and really cheap, if you happen to have or can borrow a suitable legacy (film) lens around that has manual aperture control. The adapters can be bought on flea-bay for two to five bucks, and come in most filter diameters. These consist of an EF mount on one side and male filter threads on the other side.
Front-element diopters are a good way to go for many situations, but you are introducing more glass so the optical image quality may or may not be altered. However, everyone using Canon 250 and 500D diopters has praise for their optical quality. Teleconverters are another way to go if you need to increase magnification with a longer telephoto. Closest focusing distance is unchanged but the magnification is increased by the factor of the teleconverter (ie, 1.4x or 2x the original magnification - my 400mm f/5.6L goes from 0.12X to 0.17X. The longer the telephoto, the harder it is to get high magnification.