A famous brand of Loperamide is "Imodium" which I think you're already aware of. Counter diarrhoea tablets.
Personally, I ALWAYS buy a map when visiting new places, particularly overseas. Couldn't get one in Homel, Belarus - perhaps there wasn't enough demand. Too skint for souvineers and no space to carry them although I like the idea of buying area idiosyncratic kit that's useful when back home.
Perhaps a few items of kit that are more compact than the norm. In the cases where a tourist is carrying too much weight/bulk and wants to slim down their kit so as to lighten their load - or carry souvineers!
An example, perhaps, would be someone arriving with standard sized mess tins AND a waterbottle: if you sold collapsible waterbottles or waterbottles that fit into a metal mug, I'd expect that would be tempting to many. Brew kits, small resupply bits, too?