Unlike previous answers, this one does only exactly what was asked: the reverse of C-u C-SPC. I find it the most useful.
(defun unpop-to-mark-command ()
"Unpop off mark ring. Does nothing if mark ring is empty."
(interactive)
(when mark-ring
(setq mark-ring (cons (copy-marker (mark-marker)) mark-ring))
(set-marker (mark-marker) (car (last mark-ring)) (current-buffer))
(when (null (mark t)) (ding))
(setq mark-ring (nbutlast mark-ring))
(goto-char (marker-position (car (last mark-ring))))))
Comparing to the answer by scottfrazer, this command has a subtle difference of how it moves the cursor and mark, which more accurately mirrors C-u C-spc, and it does not require that the previous command was a unpop/pop-to-mark-command.