Firstly, I would strongly not recommend a day trip to the Algarve.
A return trip to Paderne where the A2 Toll Motorway joins the Algarve Spinal Motorway, (A22/Via de Infante) would take 4 to 4,5 hours dependent on stops and you have to pay the tolls.
From Paderne, you then have to decide where you are going in the Algarve. At present, there are substantial ongoing roadworks in Lagos and I do not recommend visiting us by car until at least the end of 2009. Albufeira and Vilamoura are tourist traps and Faro is the admin capital. Tavira and Silves are worth visiting as is the beautiful west coast between Praia da Luz and Sagres and Sagres and Aljezur.
In summary, a return trip to say Tavira would take +- 6 hours not including stops.
This does not leave a lot of time to relax.
I would suggest that you might be better off making a day trip to say Porto or Coimbra.
