Just don't bother with the long ride on the Friday, you'll only leave yourself feeling flat for the race.
Do an easy hour just to check that the bike is set up ok after travel, anything more would likely be counter productive.

By all means do a swim to get used to the water, and an easy run if you feel that it would help.

I had my best sprint race the day after doing a 5 hour ride. Hurt like hell but it was fast.

I biked 162km on Monday, with a short run-off. I swam last night (Tue), and it felt very hard holding my usual interval pace. Unless the Half means nothing to you but training, you're going to be in a recovery 'hole' for a few days, and might not enjoy it as much.

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29 years and counting

Depends on how long, how hard, and what importance that Half is compared to your IM? How well do you usually recover?

FWIW, two days before a (relatively) important Half, my coach prescribed 90 minutes including 4x5min of above-threshold climbing. I felt great during the race.

Three weeks out from an IM is a classic time for a training camp or other big push. My team's camps often include what's basically a HIM-level simulation, sometimes with different distances to train specific points. Often on the last day - after two days of very hard training. If the IM is your absolute goal and you're willing to sacrifice the Half a bit, the extra volume might be a pretty good weekend to start the taper period.

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ZONE3 - We Last Longer

thats what I'm thinking - as its 3 weeks till the event, i've not got a plan for a taper as I've not done an event in a very long time - so I need to think that bit through but this would be my last opportunity to get a weekend in

I can also spend all day every day with my feet up as I won't have the wife and baby with me - so I may also get some sleep :)