Island day trips from Dubrovnik

A friend and I are on a short (Mon - Fri) trip to Dubrovnik in late August (yes, the most expensive time but my friend's a teacher so not much choice). We are very keen to take one or two day trips to the islands. We are staying not far from the port in Dubrovnik. In high tourist season is it necessary or advisable to book ferry tickets before we fly out, or are the ferrys regular where you just turn up and pay, like going on the bus?

Also on a safety note - we thought about visiting Kolocep or Lokrun, as we've heard they are supposed to be beautiful. But we've been warned some areas may still have mines around, are these islands risky areas? If there are any young female travellers who have been to the area, how did you find it in general, any places in particular you'd want to avoid?

I was in Dubrovnik around that time, and I caught a boat out to Lopud for a day trip. No booking needed, I just turned up at the dock before it was due to leave.

It was a great day, I walked the length and breadth of the whole island. Definitely worth doing.

No mine presence on Lokrun or Kolocep: Croatian Mine Action Center

Boats to Lokrum are frequent and depart from the old port in old town.
There are also day trip excursions to Kolocep and the other Elafiti islands from there.

Otherwise, you could check the jadrolinija website to see ferry schedules from the main port.

I was there till Sunday & you didn't need to pre-book.