does it have loads of shops etc and do you think it is worth it?

Yes, it does have lots of shops - but why waste a day shopping in such a lovely city? It's a European capital city and a great many of the shops will be little different to what you'll find on your own high street or in London. It's unlikely that the prices will be that much different either. On the other hand it does have a long historical heritage and some great places to visit which are unique and which you won't find in any other city such as the the castle, the monastery and monument at Belem, the amazing elevators that link the upper parts of the city with the low level parts. London has loads of great shops too but I think any first time visitor on a day trip would be daft to head to Oxford Street and nowhere else and I feel that the same applies to Lisbon.

Only you can decide if a day trip is worth it taking into account your travel time etc but personally, I think that if the main purpose of your visit is to go shopping, and expecially if your are staying on the Algarve, then I would say it isn't. Go to somewhere like Portimao and you'll still have a great shopping experience - fantastic shoe shops from memory - with a lot less hassle than going to Lisbon. On the other hand if you want to visit the historical sights, sit people watching over a leisurely coffee at a boulevard cafe and experience a very vibrant cityscape then I think that it could be worth it. I first visited Lisbon on a day trip from the Algarve and, yes, it was a very long day but well worth it with, so much to see and do and it made me determined to go back for a longer visit which I did later on. But I never stepped inside a shop either time!

SM