Let's take out Kadoorie Farm for easy walker! (as most great view require steep walking to top of Kwun Yam Shan, 546m) Yeah, try the Nunnery at Diamond Hill!
Hong Kong Park, Mai Po Marsh...how about get your mum an amateur lepidopterist (person on butterflys). Why not contact this organisation below on whereabouts of butterflys? (mostly on Lantau)
www.hkls.org - Hong Kong Lepidopterists' Society
And if she not horrified by monkeys - go to Kam Shan Country Park on Tai Po Road (on the way to Sha Tin by riding KMB bus 72, 81 or 86B)...without plastic bags. Those racals think they are foods from PnShop or Welcomes!
hope my ridiculous ideas help.