In reply to GLUF:
+1 for coffee if you are tired, it generally helps you train harder in all sports (I wonder if it's banned from climbing comps like other sports)
Otherwise it doesn't matter that much - bananas, chocolates, flapjacks etc
I've never felt that nutrition was the limiting factor, especially for indoors climbing and bouldering.
Having said this, if i have forgotten to eat anything sometimes I can feel my blood sugar dropping low during the session so I just have a chocolate and it's all good again