Best Day Hike in Iceland?

The Snæfellsnes peninsula has some nice hikes around the coast, under the gaze of the glacier Snæfellsjökull. If you're doing the Golden Circle, Þingvellir National Park is full of easy hikes with plenty to see! There are information boards near the visitors centre on Road 36.

The Hveragerdi hot river trail is beautiful and when we were there it was mostly traveled by very friendly locals. Great to do early in the morning to avoid crowds. There is some steep incline every now and again and some muddy/water crossings but it is really nice. It's only 40 minutes from Reykjavik.

I'd say Skaftafell National Park. You can take in a waterfall on the way up, plus hike north for a few hours and enjoy the scenery. If you want to take in a weird landscape, there's always the pseudocraters near Kirkjubaejarklaustur. If your parents are really active and fit, you could also spend some time meandering around the trails near Landmannalaugar. I'd say that's the most difficult (and hard to reach) of the bunch; if you go there, take a bus. You can drive to the others easily in a rental car.