I use a 2 litre Karrimor bumbag, which I bought in a sale about 10 years ago. It's plenty big enough for parkrunning, orienteering etc, as it holds keys, some coins /notes in a ziplock bag (for post parkrun coffee or post orienteering beer if on bike, not driving), credit card and / or debit card, parkrun barcode, FRA and British Orienteering membership cards, packet of jelly sweets and fruit puree pouch as hypo treatments, blood glucose meter & insulin pen. I don't carry all items for all runs / events, and can supplemented with a camera and / or small waterproof jacket, both in velcro fixed pouches, which can be attached to belt of bumbag.
For orienteering in forest areas, full body cover (long trousers and long sleeves) is sometimes specified, for protection against brambles, nettles etc. However, there is no "no nakedness" rule like in triathlon transistions, so the places where orienteers park their cars or bikes can be entertaining places. I try to remember that not all parkrunners are orienteers, when changing at tourism parkruns, when not heading home to a shower