Seal Sanctuary, Gweek
The tickets may seem a bit pricey, but when you consider that you are supporting the vital work the centre does in rescuing and rehabilitating injured seals and sea-lions, it doesn't seem quite so bad.
There is lots to see - make sure you make a note of the feeding times when you go in. There are kiosks serving snacks, a children's play area, space to picnic, souvenir shops and of course the hospital where the injured pups and seals are treated. There are also woodland walks for the more energetic.
Look out for money-saving coupons in the West Briton when you are down in Cornwall.
Details at http://www.visitsealife.com/Gweek