Nagaokakyo (長岡京市)
光明寺の「ビャクシン」
Fantastic temple, first time ever visiting this spot, which is hard to believe considering how well-known it is. I guess it is somewhat difficult to access. We weren't actively searching for the K200 signpost, but came across it out of pure chance during a much longer walk.
ゲンジボタルの生息する小泉川
The river itself is very easy to find, but the K200 signpost is slightly more difficult. We started at 西代里山公園 (note that 西代 is read as にしんだい here). This is a popular park with the locals, and the parking lot seems to crowd up quickly. From the park, walk along route 79 until the overhead expressway crosses with the river; from this point walk along the river on the west/south side, and you can't miss it. Unfortunately no fireflies during the day.
長岡天満宮
Wonderful shrine with a rich history. Easy to find the K200 signpost. We enjoyed walking around the numerous paths in and around the shrine, and were interested to find a softshell turtle (what I would call スッポン with my limited knowledge) walking along one path near a fancy eatery. Perhaps it had just escaped from the kitchen.