hey Au7hier, I went to Barcelona in May to watch the CL Semi and the Derbi. got my tix through www.spainticketbureau.com. for the CL Semi I got tix in the upper level, but for the Derbi I got tix on the 2nd level. the earlier you book the better chance you have of getting great tix.
one place that Woobs didn't mention is the Castell de Montjuic ... really amazing fort, great view, great for pictures.