Just make sure everything works as it should. These old explorers are very common, so a fully functioning vehicle will set you apart from a lot of other used Explorers, and other used vehicles too. Check to make sure all of the interior buttons/knobs work, replace blown bulbs, hose off the frame and under body, repair and rips or holes inside, and track down any odd rattles or squeaks. Anything that a buyer can use to talk you down in price (needs tires, brakes, exhaust, etc...) can be replaced by you, but you will likely not get back what you put in. However, having these types of things done can attract more potential buyers.
Upgrades and modifications almost never add value to old used cars like Explorers. Keep it as stock as you can, and have service records handy. You can dump $3k into a $1.5k car, and you will still only have a car worth $1.5k. Too many people don't understand this concept.
Cleaning it and making it look good won't add to the value, but it could make the selling process a hell of a lot easier. If you were looking at two similar vehicles to buy, would you rather buy the one with old McD's bags and candy wrappers in the back, or one that is vacuumed, washed, and looks like the PO took care of it? A little bit of Windex, vacuuming, and Spot Shot for stains can go a long way. And a simple wash is better than showing up caked in dirt and road grime.
