As others have said, Portland is fantastic for seascapes and the views towards Chesil Beach are superb. Portland has a peculiar feel to it, you`ll see what I mean when you get there!!
Abbotsbury is a must. They have a famous Swannery there, some sub tropical gardens and the view towards Portland is again superb.
Weymouth Quay is very popular as well as Nothe Fort overlooking the quay.
To the East of Weymouth is Osmington Beach popular with the Lee big Stopper,long exposure photographers, Rocky foreshore Portland as a backdrop etc.
To the north of Weymouth is Ridgeway Hill, some pretty villages on the way up (keep off the main road) with views looking at Weymouth and Portland to the South, Dorchester to the North and Western Dorset to the West.
If you can get out of Weymouth then anywhere in the Purbecks your going to find great for landscape work, particularly Durdle Door, Lulworth Cove and the derelict village of Tyneham.
If your going during the Olympics then expect some serious queues and Ive heard Portland will be cut off.
Hope all this helps, I`m sure others on here will have some great suggestions too. I hope you and your family have a really nice week and get some quality photo time in.
Cheers
Gary