I guess it depends on the type of event. At a festival you really are there for fun. Like the suggestion box idea though. I do actually prefer to write down feedback while there and then as opposed to filling in a digital survey later. When I stay in hotels, I do usually take the time to pop a note in the suggestion box (but this is usually because I want to suggest an improvement rather than praise them!).
We are looking at people rating each band after they have played too. This is something we do at another event I am involved in where you register your email on arrival and they ping you after the band has finished to do a short survey on the band rating them on 5 elements. This is for a battle of the bands competition. I am looking at expanding that at my festival as most people have our festival app so it should work quite well - hopefully!
Love it Krys - best example of an evaluation form I have seen and perfect for capturing both Quant and Qual data to inform decisions. I will be shamelessly stealing this b.t.w.