Here are three suggestions for excursions on the free afternoon, all permitting to be back before dusk. All trips involve taking a public bus for at most 15 minutes per way.
(1) Waterfall:
Type "Geroldsauer Wasserfall" into Google Maps and select "Route" and the symbol for public transport.
Follow the instructions and take bus no 204 direction "Malschbach" from stop "Augustaplatz" in front of the Kongresshaus (same side of the road) to stop "Malschbach" (recommended) or bus X45 direction "Nationalpark Ruhestein" from stop "Leopoldsplatz" to stop "Geroldsau Malschbacher Straße" (less scenic path). I describe the path from "Malschbach": Cross the street at the parking lot "Waldparkplatz Geroldsauer Wasserfall" and follow the path recommended by Google Maps (or follow the signs) until you are near a house. Here the signs recommend two options, you can go on a roundtrip clockwise or counter-clockwise. I recommend the former, for this take the path closest to the house and follow the path in the forest down to the creek. There is a wooden bridge over the creek and then you walk upstream with the creek to your right. At the waterfall you can go over another bridge to the other side and walk back on a different path reaching bus stop "Malschbach". The roundtrip walk time is around 60 minutes. Walking back from the waterfall to Kongresshaus takes 80 minutes and gives you the opportunity to see Lichtenthal monastery on the way, consult Google Maps for this option.
(2) Merkur hill:
Type "Merkurbergbahn" into Google Maps and select "Route" and the symbol for public transport.
Follow the instructions and take bus no 204 or no 205, both with direction "Merkurwald".
Bus no. 204 leaves from Augustaplatz on the opposite side of the street. No. 205 leaves from Leopoldsplatz (Luisenstraße). From bus stop "Merkurwald" take the cable car uphill (€3.50 one-way and €6 roundtrip) to see Roman excavations, climb a tower, or take a coffee in the restaurant. You may consider to walk down in 50 minutes to the bus stop or in 75 minutes to Kongresshaus, following Google Maps.
(3) Castle ruin "Altes Schloss zu Hohenbaden":
Type "Altes Schloss zu Hohenbaden" into Google Maps and select "Route" and the symbol for public transport.
Follow the instructions and take bus no. 244 direction "Gernsbach Bahnhof" from bus stop "Leopoldsplatz" to stop "Ebersteinburg Kapelle". (If you take bus no. 204 you get out at a different stop and approach the castel from another side with a slightly longer walk.) From "Ebersteinburg Kapelle" you walk 30 minutes to the castle, just follow Google Maps. You can freely walk through the ruins. Take one of the indicated paths to "Batterfelsen" cliff, you'll be there in 10 to 20 minutes. The way back to "Ebersteinburg Kapelle" takes 25 minutes. Or simply walk back to Kongresshaus in 45 minutes. This saves you the waiting time for the bus.
