I've been to Lisbon and to Ljubljana.
Personal taste - but I liked Lisbon...but LOVED Ljubljana. I've been there four times. Slovenia is just a cute little undervisted country with bit of Austrian, Slavic, and Italian influences mixed together. Granted, Ljubljana is not exactly undiscovered anymore, but it's still not on many American tourists' agendas. It's just not on the way to/from anywhere else, really. That's one reason I like it.
I last visited Ljubljana almost two years ago. I visited Lisbon for the first time last May. It's certainly a city with character, but it's also mobbed with tourists now, and the city seems almost unable to handle them all. There is lots to see, way more than in Ljubljana by far, and the public transportation - the trams and buses - can barely keep up.
Ljubljana is more about charm - and it's tiny compared to Lisbon. It's got a tiny little river running through the middle of town, beautiful architecture, a real castle on the hill - but it's still a vibrant place, a university town.
There are neat little towns near Ljubljana and some neat attractions. Lake Bled is the most obvious tourist stop, surely the most highly visited spot in the country. It's quite built up. Lake Bohinj nearby is much bigger and less developed, more of a "back to nature" experience. Many people do the "Julian Alps Loop" drive up into the mountains via the Vrsic Pass, up a windy road with many hairpin turns, and down into the Soca Valley. I also enjoy the drive from the town of Skofja Loka (between Ljubljana and Bled) to Bled through the mountains, via Jamnik and Kropa - very pretty with some neat stops here and there.
Slovenia has a tiny coastline compared to Portugal's for sure. The little seaside town of Piran is a real gem. The Venetian bell tower makes its heritage obvious. The area is still officially bilingual Slovenian-Italian. You could consider adding a dip down to the Istrian peninsula in Croatia from there - with 7-8 days, you would probably have the time. Rovinj is another delightful seaside town, old (still active) fishing village, though it's also become a tourist magnet. Things should be pretty busy there by June; I visited in early May when it was quieter.