Visiting Þingvellir in different seasons will give you completely different memories of the place.
Here the famous waterfall, Öxarárfoss, falls free and unhindered in high summer:
And here is looks like it is carved in ice.
Actually, this is just icy armour - the river still flows underneath the ice. It doesn't take much frost for it to freeze over, only a few degrees, but it does take a number of days for the ice to get this thick and cover it completely.
Here is the Almannagjá:
In the noonday glare of a summer's day:
...and on a winter afternoon