Radipole Lake is not actually a lake in the accepted sense as it is not completely surrounded by land. It lies near the mouth of the River Wey in Weymouth and has extensive reed beds along with scrapes that have been made to make it more attractive to waders.
Despite being in the centre of a large town Radipole is a superb place at any time of the year. In winter there is a wide variety of visiting water fowl, unusual gulls often turn up and it is one of the best places to see the illusive bearded tit. In summer and autumn add flowers and insects (especially good for dragonflies and butterflies) along with nesting species of bird including bittern, marsh harrier, kingfisher and cettis warbler. I have also seen an otter there.
Sadly the RSPB shop has now closed but you can get a good views if the lower reaches of the river, reserve information, a cup of coffee and even a cream tea in the visitor centre. There is a parking charge as you have to park in the neighbouring council car park. Watch the car park in winter because it gets quite puddled. That should not be a problem for you but lots of gulls (including mediterranean gulls) like to stand in the puddles and they can be reluctant to move, even for a passing car.