I've done a few things for the inside of the FJ to make it easier to use on trips and while wheeling.
Rear drawer to keep all the gear: Better Built drawer with a top I made so the dogs have somewhere to stand.
Also took out the back seat since no one ever used it and made a flip up rear tray. The tray uses the seat mounts to install so it folds up just like the seats. You can put some jackets or a jump box underneath and there's tie-downs to mount my fridge and chuck box.
When the wife and I travel there are a lot of times we want to stop real quick and grab a bite to eat and there's nowhere to do that. I had OrangeBoxx make a rear storage rack to hold odds and ends and it folds down into a table. Really well made product.
Gotta keep an eye on everything so an Ultragauge and a Dometic fridge monitor are on the dash.
Springtsail Solutions dog cage is a great product too. Keeps everything in the rear, dogs or gear, and gives you a place to mount of some recovery stuff.
I've also added LED interior lights for better viability and a bunch of power slots from Blue Sea (cigarette plugs and USB ports). Heated seats were a nice addition (wifes favorite). One of the best things I've put in to make long road hauls more tolerable is an in-dash DVD player. Often times you find yourself driving around with no Wi-Fi and no good radio channels so the ability to slap in a movie to entertain yourself and a passenger is clutch.