In mid-Sept. we are winding up a tour of Croatia (Zagreb, Plitvicla, Split, Korcula, Omis)ending in Dubrovnik. We have Sunday morning til Monday evening (flying out of DBV early morning Tuesday) to play with. We love the outdoors - hiking, kayaking, biking. That time of year will there be a daily ferry to Mljet, and would you recommend going there overnight? We're open to other suggestions, including just staying in Dubrovnik.

You will likely not want to spend any less than 2 days in Dubrovnik--words cannot do it justice.
I suggest you get your outdoor activity fix by hiking in Plitvice, rafting in Omis, and a little bit of everything in Korcula.
Check ferry schedules at jadrolinija's website, but I believe an excursion would be your best bet for a Mljet visit if you do go.
Lokrum--just offshore from Dbv.--is a nice alternative for swimming, walking, and views in lieu of a long day trip to Mljet.

We were in Dubrovnik last September and really enjoyed it.

A day trip to Lokrum is wonderful. Be sure to find the lake (it's by the monastery) and spend some time there. It's quiet and peaceful. You can also hike up to Fort Royal for wonderful views.

We also took at day trip to Mljet. I'd recommend staying a night there. It's a 2.5 hour ferry ride and we wished we had more time. Rent a car and drive from one end of the island to the other (ending at the national park).

Dubrovnik is stunning! I agree with , you'll want at least 2-3 days in Dubrovnik. However, here's a quick list of the things we enoyed.... walk the walls, have a drink at the Buza bar, check out the monastery, have dinner at Renassiance or Poliskar (by the harbor), or kayak around Lokrum.

Enjoy!

We were in Dubrovnik last September – the town is wonderful and you can easily spend 2 days there. Definitely walk the walls – this will take about ½ a day. Spend the other half of the day walking the narrow streets in the old city. Don’t miss Buza Bar and the views.

We took a day trip to Cavtat – the boat trip there was only about 45 min or so, and it was a nice change – while Dubrovnik was still very crowded in September, Cavtat was almost empty, quiet and relaxing. Try it, you will love it.

For short excursions from Dubrovnik, I also vote for Lokrum Island and Cavtat.

The islands of Lopud, Kolocep and Sipan are idyllic and nearer than Mljet. Jadrolinija's ship (www.jadrolinija.hr)runs several times a day, so you can do a half-day trip.
The fast catamaran to Mljet is run by Alpex (www.alpex.hr).

For Dixiland or anyone else who might know...please, what is the sea like in September? We get a bit motion sick, and are trying to decide weather to take a bus (which might also be a problem) or try to line up with a ferry from Split to Dubrovnik. We won't have a car until we get to Dubrovnik and are staying in Molunat (anyone stayed there?). Thanks...!

I'm leaving for Croatia on June 23 for 15days...Flying into Dubrovnik and making our way up to zagreb then back home. Any advice about rental cars? Should I keep a car the entire trip? Any must see spots? I plan on Hvar. Split,Zadar, The lake region.
Thanks for any advice.

I can't predict the sea conditions, but I think rough seas are primarily a problem in winter. A good thing about early-mid September is that it will be warm enough to stay on the deck more of the trip. I only felt slightly seasick when I was inside.
Even though it's also a risk of motion sickness, I'd take a bus because it's a much quicker way to get there and still scenic. Every intercity bus I've been on has been comfortable. Take dramamine just in case, because the roads do get mountainous.