+3 to just getting out there!
No trip is too small, or too big ... I started lugging my kids on hikes in the baby carrier when they were tiny things, took them on picnics and little walks and fishing trips and car camping when they were toddlers, started doing winter overnights in the yard when they were in pre-school, did first backpacking trips (they carried their clothes, water & personals, I handled their sleeping stuff) when they were kindergarten-ish, first trips where they carry their own ~12 pound frame pack with sleeping bag & pad before first grade ...
They started snowshoeing at three, nordic skiing at four or five, alpine at five or six.
They're six and eight now; we spent five nights out together in Baxter SP last summer ... daughter has hiked 30-something of the NH 4K's, son has decided he wants to hike the whole Long Trail (we're ~60 miles along at this now), and they enjoy getting out whenever they can, any time of the year.
Don't lose sight of the fun of 'easy' outings. Last Friday night on a lark after school, finding it was going to be a mild night (+20F) we pulked our stuff up the hill out back, set up camp, invited the neighbor kid up for a starlight marshmellow roast, walked her home, and returned for a night of easy winter camping. We were back home with all equipment stowed or hung to dry by 8 in the morning.
Figure out what they like ... my daughter likes streams and views, whereas my son is more interested in ski area machinery and wreckage of old settlements. They both like steep scrambles and abhor monotonous sections through the woods.
Edited to add -- anything on skis gets extra points, ditto anything with swimming.
They're as capable now as a good many of the adults I know, and it's gotten to where I don't do very many trips without one or the other of them along, if not both. It's the best time we spend together.