Blimey… where to start - the aftermarket accessory market for the Mavic Pro is just huge, isn’t it?
Which is good…
Beware! I got a bad dose of accessory-fever. I think I may still be suffering with it too.
Let’s hope my wife doesn’t see this thread 
I bought one of those round, folding, landing pads - with bit of a knack it folds itself down in to a small bag (same thing as yours?) and I think this is probably the most used accessory. Maybe not the best, but certainly the most used. I think I probably use it on 99% of all my flights. And it doesn’t weigh anything, easily fits in the rucksack too:
I’ve also sold the supplied case / shoulder bag (did you get the flymore combo?) and I replaced it with a Smatree D500 case, which is probably the best thing I’ve bought so far. It’s a semi-hard case, exceptionally well made. I think it cost about £50 quid or so on Amazon (will try and find a link for it) but I sold my DJI case for £45 on eBay, so I nearly broke even! Here we go: https://www.amazon.co.uk/dp/B01MY920DU
Other things I’ve bought…
A gimbal cover and lock, all in one. Again, used every day. This is almost identical to the supplied dome cover, but includes two prongs which lock the gimbal in place, removing the need for the OEM gimbal lock. Very easy to put on and take off, £6 well spent - https://www.amazon.co.uk/gp/product/B071RVC2VW/
I’ve also bought a landing gear leg extender set. But I’ve never used it yet 
Another great accessory, some Strobon CREE ultra bright LEDs. These are tiny little LED lights, complete with their own (rechargeable) battery. They clip on to the legs of the Mavic (requires clips) and flash like a strobe. Soooo bright… They really help with finding the bird in the air. Often is the case I look down at the screen, then look up and WTF! It’s gone!! I literally can’t find it in the sky! And this is in VLOS too. Crazy… so yes, I have two of these, one on each rear leg, one facing forwards, one facing backwards, so I can see it at any angle in the sky. Bought them from Flytron here in the UK. Not cheap though… http://www.flytron.com/led-systems/322-strobon-cree-standalone.html#/color-white
I also bought a couple of SanDisk Extreme 32GB MicroSD Cards.
And a Hensych Hood - which to be honest, was £15 quid down the drain. First up, I fly with an iPad Mini 2 as my screen, it’s REALLY hard to see in broad daylight. The iPad fits perfectly in the Mavic controller too. I bought this hood, it didn’t really fit properly along with the controller at the same time so I cut a few small corners out which helped with the fit. The only problem then was it got in the way of the joysticks. Well, not in the way as such, but I couldn’t comfortably hold it. Coupled with the fact it didn’t do much to prevent the glare. It’s in the bin now. As too is the Brotech screen protectors I bought. Advertised as anti-glare, utter rubbish.
I also bought an Anker Powerline+ USB cable for the iPad - cost £10 quid but is beautifully made, never had a single disconnect with it.
Another great gadget for flying with the iPad was a MavMount - https://www.mavmount.com - Horrendously expensive for what it is, plus shipping to the UK, plus getting caught by customs. But again, sooooo well made! I have a v1 for sale too, check the for sale section here. I’m open to offers 
I’ve got a twin Getterback setup too, on a custom 3D printed bracket - So I had a twin Getterback bracket 3D printed - #2 by PingSpike
Um…
I think that’s about it (so far!)
I told you this was addictive!