Midsummer Common: from Mars to Earth and on to the Sun. A trail along the River Cam, beginning at Midsummer Common, showing the relative distances between the planets' orbits, marking each planet with a coloured arch.
Reality Checkpoint - so called because it supposedly marks the spot, halfway across Parker's Piece, where the bubble of Cambridge University ends and the "real world" of Cambridge city begins. I believe there used to be a plaque inscribed "reality checkpoint" on the lamp-post, but that is long gone and the words are now rather crudely scratched into the paintwork.