Wife = toilet on board is a must. Buy a rod holder mounted BBQ with a lid for cooking dinner. Take those 10l 'water in a box' for drinking as they collapse to nothing when empty.If really getting into it get a 2nd rear canopy made with poles at the back so you can walk around in the cockpit under it when anchored up. Take good cleaners etc so the boat can be cleaned of fish smells before you stop for dinner and bed. Rechargeable mini-vac is also useful. Make sure that you have good lighting on the boat and good batteries. A slightly oversize anchor with a bit extra chain will help you sleep better.

If it was me I would anchor in shelter but have a smaller anchor/grapnel aft to hold things steady, maybe bungy it.

To show what's possible with a bit of planning and an adventurous mindset, check out this guy on YouTube - overnighting in what looks like a 5m or so open dory.

Wife and I overnighted many a time in our 5.5m commander.
As mentruioned, portable toilet a must.
 Set up a 2nd baitboat on star fittings for a cartage gas buirner.
 Make sure u know the breeze direction for the WHOLE night..
 Park up in a small bay...
Anita bay..bottom end waiheke.. either Nth or Sth cnr depending on breeze... a favourite..
and mark the postion in the afternoon... fishing maybe good on dusk so need to find the spot in the dark..
 Try and choose a bay in lea of land where the valley that comes down has a lot bush on...the bush breaks up the breeze before getting to you.
 KNOW what the seabed is like.. just because a bay maybe marked as suitable ancohorage, it may still have a rock bottom...
 Know the tide height at mooring time and the change of height to allow for amount of warp out.
Put the drift warning in on the gps ..
Take ear plugs.. hull slap...

Rem the water is about 21/22 deg at the moment ...its not cold as one would expect... camping in a tent is colder..

Choose your days/weekends where light winds, reasonable to good forecast.. if possible on an east facing bay...wake to the sunrise coming up over the coromandals...
fire up the engine, idle out to the reef while the billy is boiling and drop a line on the drift..

I have a 5.7m fibreglass boat and a mate and I overnight on it often. The cabin is reasonably sized and I got a back-drop made that fixes over the cockpit for overnighting. My last boat was a fibreglass 4.7m and we spent plenty of nights in that too! 

As to cabin size.. 

My first couple of boats were aluminium because despite wanting an alrounder I kept getting told by forums and salespeople that I will mostly use it for fishing. (Ignoring what I was actually asking for). We didn't overnight simply because the boats didn't enspire us to go overnighting.

We then bought a fibreglass boat as a family and overnighting boat and now actually use it as a family, taking friends and overnighting boat. I also use it for fishing with mates a lot more than any other boat I've owned because it handles so much more weather that the gods could throw at it.

I'm not suggesting any particular boat is better than another, but what I am suggesting is buying a boat to suit what you want it for, not what everyone else says you want it for.

another good item to take is a 5v usb fan, sit it in the cabin and run it all night, keeps the air moving and helps keep the bugs away, used one both nights we were out and never changed off of the first battery, remember to have a 2nd battery as a back up.

Glass or alloy? I wouldn't worry too much about wave slap if you're new to over nighting,you will be subconsciously awake worrying if anchor has dragged,soon get use to wave slap,if sleeping up in the bow you will here you're anchor warp creak n groan if any breeze and that will keep you awake. Am I dragging,better get up and check,warp will creak n groan through bow roller so just put a rag under warpp,30 odd yrs and I still wake up to the slightest groan,no noise then I start worrying thats when an anchor has dragged.

Glass or alloy? I wouldn't worry too much about wave slap if you're new to over nighting,you will be subconsciously awake worrying if anchor has dragged,soon get use to wave slap,if sleeping up in the bow you will here you're anchor warp creak n groan if any breeze and that will keep you awake. Am I dragging,better get up and check,warp will creak n groan through bow roller so just put a rag under warpp,30 odd yrs and I still wake up to the slightest groan,no noise then I start worrying thats when an anchor has dragged.