The reserve used to have three hides, but two of them got burned down, and there was one at Barr Loch, but I think that got vandalised too. I went to it once but it felt quite remote, but with all the grafitti, empty beer cans etc it didn't feel like a safe place to linger.
You're picking the best time of year to visit, it is very very quiet (on the water at least, but the rest of the reserve teems with warblers) in summer, but from now right through to April it'll be really busy birdwise. As a beginner you might find it an ideal reserve, because it's not a really big reserve so your view of the birds (except for the far corner of Aird Meadow) will be really good.
There is a faint path around Barr Loch, I've not been right round myself, but some people on here have.
Otter sightings have been frequent recently on Barr Loch and Aird Meadow.
At the river down towards Lochwinnoch village is a great place in summer for swallows and house martins as they fly over and under the bridge - you get outstanding views of them.
Regarding the smew, these have been turning up every winter for about 5-6 years now.
They're reshaping the reserve to flood it and bring the water closer to the visitor centre. They're trying to attract in more wildfowl like geese and ducks.
Muirshiel Country Park is just up the road from Lochwinnoch, but you really need a car to get there. In summer the hen harriers are up there, and you've got a good chance of dippers, grey wagtails, stonechats etc. There's a walk to an old mine across moorland where in summer you'll see wheatears and meadow pipits, and if you're very lucky maybe even a ring ouzel, but keep that for a decent weather day.