Although my own main interest is birds, I think if I was advising someone on a natural-history related hobby to take up, I might suggest flowers or insects. I think it’s a great virtue to look closely at the little things. You miss the real action if you tromp through the hills, admiring the view but not noticing the wild flowers at your feet.
You don’t have to pick just one interest, of course. I did much of my early birding with one of my teachers who was also keen on flowers and had a moth trap. He was the one who showed me that, if you look the wrong way through binoculars and bring them very close to something, they act as a powerful magnifying glass.