A strange question. Effectively you're asking, "Recommend me any lens as long as you think it's special."
If your main interest is toying around with lenses, why don't you just look at those cheap lenses, check them out on Flickr, buy them if you like the pictures, try them and sell them again? Lots of people seem to do that and are happy.
Regarding those M42 and FD and Olympus lenses for £20, the interesting ones from those systems are more expensive than that and already the adapters are going to cost you more than £20. That leaves you stuck with Nikkors. Enough stuff there to try: 24/f2.8, 50/f2, 105/f2.5, ... (buy pre-AI for lower prices). If you want to play around with other systems, why not those lenses that don't work on any DSLR body, they tend to be cheaper (Konica AR 40/f1.8 and so on). Or buy a M39 bellows unit and try a few enlarger lenses, those are really cheap nowadays.
Instead of looking for good lenses, you could also deliberately look for joke lenses. That would strike me as more original. With your Nikon adapter the Nikkor 43-86/f3.5 or 30-60/f4-5.6 come to mind. Make a photo series "Cat shots with dog lenses" or something.
