If you are in decent shape and have a day - ask the hotel to pack a sandwich lunch for you (keep it in your bookbag) and to tell the cabbie to stop at a supermarket so you can buy some fruit and bottled water (doesn't need to be much - don't need to speak Chinese - it's all rung up electronically).

Get a cab for the day and take it to Jinshanling and have the driver be told to wait for you at Simitai (pricing information is available on this board if searched and some of us have debated it for ages).

Hike from Jinshanling to Simitai is about 10km (I think). Much of the hike is on unrestored parts of the wall. There are few tourists and the few tout ladies who will trail you along and not bug you much can always help out (my sister felt nausea on the wall and one of the ladies took her off the wall and got her to Simitai in less than an hr with some shortcuts - my sister guiltily bought some cheapo trinkets to compensate her). But there will be no (to my recollection) t-shirt stands, jade hawkers etc. However about 3/4 of the way through, you will have to buy another entry ticket into the Simitai part of the wall (not a big deal).

It's breath-taking, unrestored (for many parts), few hawkers, no real gimmicks. It's just often a timing issue more than anything else. If you are not in great shape, I would just go to Simitai, walk around a bit, chill out for a while and go back to Beijing, but some of the best parts and views will be missed (hey it happens).

If you are pushed for time, take a cab (negotiated once again) to Mutianyu. Nice views, convenient, easy, though far more commercial.