Our vacation will take us from the French Riviera to Venice to Trieste (our first time to Trieste). We would like to take a day trip from Trieste to the Istrian Peninsula. We are not sure if we should drive along the coast and visit Umag or drive inland and stop at Rijeka. We will have a car. Which would you recommend for some sightseeing and a pleasant lunch or dinner? Or would you suggest a different place to visit in this area? We would also appreciate a few restaurant suggestions.

Thank you.

My husband just mentioned Opatija to me as another option.

Skip Rijeka. It's not that interesting. Here's info I received when inquiring about the seaside promenade near Opatija:

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i am a regular visitor to Istria/Kvarner and happy to give some information. The promenade is lovely and it stretches for 8 miles from Volosko harbor to Moscenicka Draga. Stop in Volosko harbor ( free car parking) and have lunch at Le Mandrac (one of the best in Croatia) After lunch, walk as far as you want along the promenade and when you want to return, you are only a short distance from the road from where you get the #32 local bus that runs every 15 minutes back to Volosko.
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Unfornately I didn't keep the name of the helpful poster. (We were disappointed not to be able to do this, but my husband got food-poisoning.)

Other pleasant possibilities are Porec and Rovinj -- and Piran in Slovenia. We loved the small seaside towns.

Thanks for the suggestions, Mimar. Rijeka looks like a big city so we will most likely visit one of the small seaside towns.

Piran is much closer and very nice.

As Bob says, Piran is very nice, as is Rovinj. If you are driving a rental vehicle be sure you can take it across the borders before you set out.

I would go to Rovinj and/or Pula. If you go to Rovinj, be sure to explore the twisting alleyways of the old town.

Pula and Rovinj are terrific, Opatija an Austrian empire period piece. The countryside in between looks like Tuscany.

Being Croatian, I might be a little biased, but consider spending more than a day in Istria.
I wouldn't recommend taking local roads from town to town, they are not actually running along the coast and can be crowded during the summer. There's a freeway from the northern part of Istria to Pula, with exits to every bigger coastal town.
Rovinj ("Istrian Dubrovnik" and Pula (Roman forum and other buildings) are not to be missed.
Then there's Gulf of Lim, a fjord-like bay near Rovinj, the magical Brioni islands near Pula (yes, the suits are named after them!), the ancient town of Porec... Inner Istria is beautiful - just to mention Bale with a medieval castle and a small, anything-but-ordinary restaurant nestled in it.

BTW, Piran is in the Slovenian part of Istria

Have fun!

Just checking back here and found this great information. Thanks for the sightseeing tips, everyone.

We will also check if it is okay to drive a rental car from Italy to Slovenia and/or Croatia. We will be driving it back for drop off at the Venice airport.