sincde you mentioned Ireland, I would check out the price of a self catering apratment for a week somewhere in the country and then fill the rest with Dublin

I think that Ireland would be a good choice...

fits the bill for cooler weather

stay in small scenic villages at inexpensive b&b's (large Irish breakfasts will keep you filled up for most of the day)...my favorite towns are dingle and kenmare

stay in town...catch free entertainment and meals at pubs in the evening

Ireland's best spots are it's scenery...again...free

rent a small car (no kids...don't need the extra room)...you can save more money if you think you can drive manual on the left...I would splurge for the automatic.

our first trip we saved money using the heritage pass (check your itinerary, to see if you will be near several heritage sights)

12 days would be perfect for the south and west /in and out of shannon(3 connemara, 1 doolin, 3 dingle, 3 kenmare, 1 cashel, 1 bunratty)