Get yourself a 1:25,000 White Peak map (https://dash4it.co.uk/catalog/product/view/id/9910/s/ol24-the-peak-district/ use code BASKET10 for another 10% off). It will have a download code to use with the free OS app on your phone.
From Millers Dale I suggest:
1 Walk upstream at river level to the crossing near Mosley Farm, the return at higher level along the Monsal Trail.
2 North up Monks Dale to the minor road crossing, then back via Wormhill and Chee Dale Nature Reserve (only do this if dry underfoot - Monks Dale is full of ankle-wrenching slippery boulders)
3 Drive a couple of miles to Monsal Head, park along the road to Little Longstone, walk up to Longstone moor (path next to the Packhorse or along to Great Longstone and up Moor Lane) continue north until cross-paths with a left turn to Wardlow. Take that into the village then left along road until Path on right leading to Cressbrook Dale. Down Cressbrook Dale (last bit on minor road), take path by Cressbrook Mill to cross the river at Water-cum-Jolly then back to Monsal Head along the Monsal Trail. Award yourselves a pint in the Packhorse if you get back before 3pm (or visit again in the evening).
I’ll post a few more further away later.