What age are your dc?
East Yonderton farm out by the airport does fruit/veg picking, just pay for what you pick. Can then make things from what you pick.
For slightly older ages geocaching is good if you have gps on your phone. Google if you've not heard of it.
For younger ones there's an animal corner at Barshaw Park on paisley Road West/Glasgow Rd. Has a playpark park, pond, big hill for rolling down! and somewhere to buy teas and ice cream too.
Heads of Ayr is really brilliant if you don't mind travelling a little and paying a bit more. Go on a nice day and it will be more than worth it, number 1 day out for my lot and my nephew (ages 2-11)
Depending on your area and kids ages you can get a glasgow kidz card and get cheap sports things. For example bellahouston does sports sessions where they play games etc for 2hrs and its 50p per child. Kids can also get free onto the open top tour bus with it.
Botanic gardens are free and also has a playpark.
Victoria park near the Clyde tunnel has fossil grove (330 million year old fossils)
Odeon do kids films for £2.50 per child and adults get in free. The quay one is normally 11am and at braehead at 10:30.
Scotland Street school museum opposite Sheilds Road underground is free.
Kelvingrove museum and art galleries is free and has loads of animal exhibits and run little groups in the hols.
The new museum of transport is due to open in a couple of weeks, the old one was free but not sure if there will be a charge for the new building.
Lamont City farm in Erskine is free too, small domestic and farm animals and probably best for younger ones too.
If your kids are older loads of places do one off days during the holidays, eg Glasgow climbing centre do a day of climbing and abseiling, xscape do snow camps.
Science centre is more expensice but you can spend the whole day and it's indoors which is useful for Scotland!
Blair drummond is also worth the extra, go on a dry day and you will easily be able to be there from 10am-5pm, there's a huge adventure playground, rides, pedallos, farm area where you can fed animals, sealion show, bbq area, drive through safari animals etc. Normally a voucher for a free child on website
Ayr beach has great park accross the road and apparently has new area with rides etc
Went to whitelees last summer with about 10 kids various ages between 3 and 10 and they loved it. Thinking back there's not much there but they got loads out of what there was, helped there was lots of them so just running around by the turbines was fun.
Also agree with Dean Castle.
Sure there's more but I'm off to bed!