Hobgoblin is right, the Libori fest is great fun..
If the family include children of any age the Therme [http://www.vital-hotel.de/] at the Vital Hotel Bad Lippspringe will tire them out and is good value.
Over 27 years since the good old days when I was based at Barker Bks, but we used to enjoy a day out at Mohnesse near Soest and the Paderborn Summer Festival was a must. Unless its gone down hill since then it would be great for families especially kids.
Mohnesee is always a good one. Even better if you can get the dam busters theme on CD and play it as loud as possible as your parking the car
Wewelsberg Castle is good too. It was going to be the centre of The Reich and Himmler started doing some big plans to make it so. Very interesting and not too far down the road is what is left of the concentration camp that held the workers who started the work.
Head West towards France. Get in the sea and swim. Alternativately play 'Smack My Chin' Tis easy, smallest kid bites a detonator in its mouth and the rest of you play 'Smack The Chin!' with steel toe cap Doc Martens.
Sunny day - visit the parachute centre at back of the Sennelager Ranges ranges near Bad Lippspringe - kill a couple of hours watching the parachutes, having a soft drink or two, and a snack, maybe watch some fresh out of the air skydiving videos, meet some real daredevil jumpers (could even take a tandem ride - like this young lady, of whom MORE HERE )