I did my SRP on "how soil changes with depth?" It was categorised as a 'hard' earth an enviromental experiment but I found it quite easy. I can't really give you any advice for the SRP but to choose something you like, have the resources to do and do well and that will fit into your weekly timetable. If you've got a lot of other stuff (work, training etc) on during the week, don't choose something that is going to require constant work, choose an experiment that can be conducted over one weekend or something. Good luck!