Provided you travel down by public transport I would think Lagos would be ideal for you this summer.
I refer to the public transport issue as the town centre will be a disaster this summer owing to ongoing and delayed roadworks.
You can get down from Lisbon by either long distance bus or train.
Cost will be between € 16:00 and € 23:00 per person each way dependent on which service you chose.
Buses:
Eva Transportes/Rede Expressos operate several daily services leaving Lisbon Sete Rios Bus Station / Metro Jardim Zoologico Bus Station and taking approx 3hrs45min to Lagos.
Renex operate several daily services from Lisbon Gare Oriente Bus Station to Lagos. Transit time circa 4hrs30mins.
Trains
See Cubsur's excellent site here, complete with the Lisbon to Lagos Timetable.
http://thomasallen.home.sapo.pt/travel/006.htm#times
We have several great beaches here including the 5km long Meia Praia Beach. Lagos is on the world traveller/backpackers circuit and there are great bars, restaurants and hostels.
Lagos also retains the feel of a small Portuguese town thanks to its history, town walls and location.
If you want to get out for the day you can get by bus to places like Salema, Luz and Burgau where there are also nice beaches:
