Hiya,
I'd pack some skin so soft just in case (I'm 20 miles from Biggar & we've got lots of midgies!)
New Lanark visitor centre is nice & has some beautiful walks round the falls of clyde, also wildlife centre there with falcons, otters etc.
Tinto hill is good for hill walking, as long as it's a nice day !!!
If it's thrills you're looking for M&D's themepark, just off the M74 at Hamilton is bound to fit the bill - it has indoor & outdoor rides, cafes and bars and Amazonia - indoor jungle type place, full of bugs, beasties snakes etc.
You're not too far from Glasgow at Biggar either - there's the Transport museum at Kelvingrove, Art Gallery and the Science centre is good for a day out with kids.
Edinburgh's worth a day trip too - see the castle & zoo?
Here's a link for lots more too !
http://www.visitlanarkshire.com/Hope you have a great time - the weather's not been so good lately
, but given 4 weeks - who knows, maybe the sun'll come out