there is a large free National Trust carpark just outside the village, from which there is a footpath to the henge.
there is free parking just down from Silbury Hill.
there is a reasonable layby next to the footpath up to West Kennet Longbarrow.
best time? i visited on a miserable wednesday once and had the place to myself, i've also been on a saturday when it was bedlam. if you can only visit at a weekend why not go east a handful of miles and visit the Devil's Den, which is out of the way enough you should have it to yourself.
that said when it is busy at avebury most folks don't bother with the Avenue or the Longstones, or what's left of Falkner's Circle.
i've not been up to Winterborne Bassett(?) what remain's stone circle, but that's only a few miles to the north.
and of course there's always the ridgeway, if you go up the Herepath (?) from the henge you come to it and just beyond is National Trust land with sarsens still in their natural state (grey wethers), from the ridgeway you can walk down to the site of the Sanctuary and then up the Avenue back to the henge, or take your life in your hands and walk along the main road to the footpath for West Kennet Longbarrow.
you're on for a great day out, but on our first visit we couldn't get into west kennet as some american new agers were smoking it out with incense, chanting and bongos, so be prepared for others exercising their right to do as they please at the expense of your own.
