I like to combine a hamfest with something else nearby, e.g. TNMoC.
Used to be they took all day, at least if you spent it going round and round until you were sure you hadn't missed anything. Many are over by lunchtime now, making that more practical.
MKARS used to be held at Bletchley Park, where TNMoC is located. I've been going for many years, it's been there, a school, some airbase (maybe cranfield ?) and currently a community centre. It's usually on the sunday before the bank holiday, and on the bank holiday there's HARS (Huntingdon) which for as long as I can remember has been at St Ernulf's school in St Neots. I usually see both MK and Cambridge people there as it's pretty much halfway between the two. It was fading out but seems to have revived a bit recently and is a little bit bigger than both the others.
The RSGB rally also used to be nearby at Woburn Abbey, moved to Newark now. And the Dunstable Downs rally Mike mentioned was previously at Old Warden Aerodrome / Shuttleworth Collection a bit down the A1 at Biggleswade - it grew a lot when it moved to Luton.
I'm tempted to take a gazebo, teapot and gas stove to one. Anybody got a few chairs ?