If it were me - I'd get on outside Epcot and ride to Caribbean Beach. Transfer to the Pop/AOA line to go over Hourglass Lake (if you can time it for sunset? Chef's Kiss!) Then maybe explore one of those resorts (or if you like a nice cocktail, I recommend the Pina Colava at Petals Pool Bar at Pop.) Then get back on to go back to Caribbean where you transfer back to the Epcot line but get off at Riviera. There is a lovely place to grab a quick bite and cocktail (Bar Riva) and explore because it is a beautiful resort. Then you can either get a bus back to the Disney Springs area from Riviera, or head back to Yacht Club for a bus.
You can not just ride round trip. No matter what, you have to get off at Caribbean Beach whether you are transferring to the Pop/AOA line or just want to head back to Epcot. Same if you head to Pop. Even if you don't want to explore, you have to get off and join the return line. They don't allow you to stay on at either place. The only time the doors open and you don't have to get off is at Riviera.