Lymm is quite nice and not too far from Manchester either. We went a couple of years ago and had a walk around it and the pond in the centre is really nice. We took a stroll down the canal, back towards Manchester and our son loved it on his bike. Everyone was very friendly as well, it was really nice.
We also did a bit of a scramble up to Gladdings Dam (Todmorden way) which is really...strange! It's a reservoir on the top of a 'mountain' (as our son calls it) and it has a beach up there! Honest!
It's well worth a walk up and it's totally unexpected. I know it sounds strange, but it's the only place I've ever seen a duck on the beach!
We also love Hollingworth Lake near Littleborough. I like to think of it as the 'seaside' as it has arcades, chippy, bit of a beach, play area for kids, pubs, restuarants, ice creams - everything Blackpool has :) The walk around the lake is just right and it's a great place to visit.