You will have a fantastic time! We're sailing on Iona this year and then Arvia next year. We've done Iona before and she's a lovely ship.
My best tips are:
Remember there are USB ports under the bedside lights for charging your phone etc.
Buy one of the plastic cruise luggage label covers for putting your bag labels in and attaching to your suitcases. Officially P&O suggest printing off your luggage tags and stapling them round your suitcase handle but I think you'd risk getting the labels ripped that way.
Take a couple of Dr Beckman laundry detergent sheets if you need to do washing. They take up almost no space but launderettes are free and have washing machines, tumble dryers and irons.
You can ask your cabin steward if you need more clothes hangers and don't forget to take some cash on board if you want to leave them a tip at the end of the week.
You really don't need to overpack. I took WAY too many pairs of shoes on my first cruise when all I really needed was a pair of trainers for walking round on port days, a pair of flip flops for round the pool and a dressy pair of shoes for the evening.
Iona is usually kept a nice temperature inside but you may want a cardigan or other light layers for on deck early mornings or in the evening.
The Glass House is a lovely spot for a glass of wine and people watching in the atrium.
On embarkation day if you want somewhere quieter for lunch go straight to the Olive Grove and let everyone else queue at the buffet or The Quays. That said, the fish and chips at The Quays is really good so I'd recommend that.
We tended to avoid the buffet at all costs other than for late night snacks. It can get REALLY busy. Breakfasts in the MDRs were much calmer and nicer.
Have an idea of what you'd like to do in ports but remember sometimes it's nice to just mooch about and not plan too much. What's In Port is a really good website for handy hints about pretty much any cruise port in the world!
You can just get a drink from the bar in Sindhu and it's much quieter than some of the other bars. Similarly you can get Costa coffees at The Glass House or Ripples (the ice cream counter) and beat the big queues at Vistas.