Since it's already mid-afternoon locally, treat the morning block as a relaxed late start: if you choose Bled, arrive mid-morning for a gentle lakeside stroll around Lake Bled, stop at the chapel viewpoint on Mala Osojnica for classic photo ops, and grab a kremšnita and coffee at a café near the promenade. If you choose Koper, wander the old town streets around Tito Square and visit the Cathedral of the Assumption and Praetorian Palace, finishing with a leisurely brunch at a seaside bistro on the Riva. Both options are low-key and set you up for a fuller afternoon without an early start.
For Bled, spend the afternoon taking a pletna boat to Bled Island to ring the wishing bell, then climb to Bled Castle for panoramic views and the museum exhibits before a late lakeside walk and optional boat rental if time allows. For Koper, dedicate the afternoon to the coastal circuit: walk the harbor, visit the Slovenian Coast Maritime Museum, then head to the nearby seaside village of Izola or Ankaran for a seaside promenade and fresh seafood at a marina restaurant. Travel times between sights are short, letting you enjoy both exploration and relaxed breaks.
In Bled, wind down with sunset views from the Castle terrace followed by dinner at a cozy inn serving regional fare (try the trout or hearty jota) and a final lakeside stroll under the evening lights. In Koper, watch the sunset over the Adriatic from the Riva, then savor a seafood-centric dinner in the old town and finish with a gelato or an aperitif at a bar near the harbor. Since you're not staying overnight, plan to leave with enough time for your onward journey—both towns offer compact, atmospheric evenings perfect for a short but memorable visit.