In reply to Lenin:
Criffel is nice, but be advised the direct route when you emerge from the forestry is the definition of muddy - better to head off west from the forestry to gain the ridge rather than go direct.
Cairnsmore of Fleet is another option and has a memorial to aircrew who sadly died on the hill over the years when their aircraft crashed - can be extended by walking the ridge south.
Screel hill makes a nice wee walk and you could add Bengairn to make it longer - but if you do it the paths disappear quickly on Bengairn (or at least they did for me, but then paths generally disappear quickly in Galloway)
Quite a few options down Glen trool including the Merrick as already mentioned - if the weather is poor, there is a nice shorter wander around Loch Trool.