We stayed in Ana's Belem flat for two weeks in August. It was a perfect place to explore Lisbon and enjoy the beaches of Caparica. Coming back to the flat every night and opening the windows and door to the terrace, let all the evening breeze cool us down, while enjoying the sight of the sun going down over the town with a glass of wine. From the flat it is a short walk to the famous monastery, don't queue, just go late in afternoon and you get beautiful light in the chapel thrown in for free. The pastries are as good at Chique de Belem ( around the corner of street) as they are at the famous (and forever queuing) blue fronted shop! The Berardo collection is worth a visit (again walking distance) if only for the building.
We went into town to explore which us fun on the old 15E tram (but more pleasant, AC, on the newer trams). The Gulbenkian museum was a highlight, as well as the ride in the 28 tram. At night trams are far fewer so think about catching a train from Cais de Sodre or a taxi (t follow the boat schedule so we had to wait for an hour for a 20 min bus ride. The beaches at the end of the bus ride were very crowded but luckily the little tram (Transpria) was still running. This takes you out for another 10km or so.... On subsequent beach visits we rented a tandem (there is a bike rental place on the riverfront, right next to the EDP museum) and cycled all the way to stop 19 on the tram ride. In all this journey, including the boat ride (very cheap at 1.15 each with the Viagem card, bicycle free), took about an hour (we are regular cyclists!). At stop 19, beach is emptier (even in August) and beautiful. However beaches at Cascais and Estoril are even easier to get to (but full of people) by train, just 2 mins from flat and again with Viva Viagem card, dirt cheap!
The flat has all you need and is cool in many ways, plenty of space with the terrace and comfortable bed etc. We recommend it highly!