A couple of day trips or half-day trips. Sometimes fairly touristy things, other times things a bit more off the beaten track. I research the area on the internet before we go and draw up a list of possibles, both fair and wet weather alternatives. Some years we don't use the list much at all, we just improvise, but it's there if we need ideas.
Loads of walking, especially coastal walks. Have done ever since the kids were small, we take a pedometer to keep an informal tally of how far we've walked. The year our youngest was 5, we walked 44 miles over the week, and we didn't walk all the time by any means!
We just like to explore so we get out and about whatever the weather. If the weather is good, the children like some solid time on the beach, or more precisely in the sea. Otherwise, we go rock-pooling or crabbing or fossil-hunting. We hunt out less crowded beaches, sometimes if you walk a little bit further you can get away from it all.
We explore the local area fairly thoroughly. If there are any museums or heritage centres we'll go to them. We'll walk to/from local villages, have a look round, have a drink and a cake or ice-cream, then walk back. Any little seaside towns around, we'll visit those too. We love finding craft shops, bookshops and lovely local foodie shops. Any 'different' shops really.
For lunch we either take a packed lunch or we head back to our cottage for lunch and a bit of a rest. In the evenings, either after tea if we've cooked it, or before going out to eat if we're eating out, we have an hour or so in the cottage, again just relaxing. Perhaps have a shower, read, write postcards, the children all keep holiday diaries - even now they're getting 'old'!
After eating we always go for a walk, unless the weather is really dreadful. Sometimes we have an evening swim, that's one of our favourite times. Then back to the cottage where we laze around, read, play cards or whatever until bedtime. We all do a lot of reading on holiday.
We don't socialise at all, that would ruin the holiday for us! We wouldn't go on holiday with other families either. The only exception is once or twice we've invited my Dad along with us, he lives on his own and wouldn't otherwise have a holiday. He's no trouble, he just goes along with whatever we want to do!