You may find that you will be so dehydrated while climbing the Great Wall during summer and will rush to buy the bottled water from the local traders around the wall.
Be aware that they tend to sell you "fake" mineral water which is actually tapped water in empty bottle collected from the bin. Most of them will freeze the bottle into ice and sell it to you as if it is a mineral water from the water plant.
Take a close look at the bottle cap and make sure that it has the standard leak-preventive ring below the cap. Otherwise, that is a re-use bottle.
Unique Suggestions: Carry one bottle water with you before you leave the city.
Fun Alternatives: Otherwise, don't buy mineral water but get other types of drink which are less likely to be fake, for example Green Tea, Ice Tea, Coke, etc etc.
I once visited the Great Wall at Badaling with my sister. As I had forgotten to take any food with me I wanted to buy some cookies and a bottle of water at a state owned shop. They had the prices shown in the display, but it took them 3 times of counting until they got it right.
At one of the souvenir shops I saw a hat, which I liked. I bought it for a good price. But I did not check the change. A few minutes later I discovered that I had got some Taiwanese papermoney as change. I returned to the shop. When I waved the money at the shopkeeper, she changed it to the correct money without any discussion. I think she was very afraid, that anybody would see, what she tried.
Unique Suggestions: Be carefull at any tourist spot in China! Never trust a shopkeeper when it comes to money. Sometimes I thought it is maybe a kind of sport for the Chinese to see, how much money they can get out of a stupid western tourist. But they respect it, when you show them that they cannot cheat you. So I made it my sport not to be cheated. I did not always succeed but most of the times.
Fun Alternatives: Take your own food and water with you, which you bought at a shop downtown.
Avoid Badaling which is the most popular and well-restored part of the Great Wall and hence it means it is heavily touristed. Scores of tourists are disgorged by the bus-loads.
The 'amusement park' is filled with stalls selling tacky T-shirts, 'I Climbed The Great Wall' certificates and kitsch souvenirs for you to splurge more money on.
Fun Alternatives: Instead, go to Huanghua Chang Cheng (Yellow Flower Fortress) which is a wild, crumbling, barely-restored but still rather well-preserved part of the wall with beacon towers and broken steps overgrown with foliage. There are at most a handful of tourists around.
Way too many people at the BaDaLing section of the wall. Don't go there.
Unique Suggestions: Well, if you are stuck going there, at least go on the darn wall. You did make the trip. But try to avoid the street vendors. Most of the stuff you can get cheaper in markets in the city.
Fun Alternatives: Go to some less conjested sections of the wall, check a tour book like Lonely Planet's China book.
If you go to see the Wall in Badaling, the best way to go up is NOT BY FUNICULAR, but by foot . Is cheaper and you see more, the funicular goes too fast and you don't really enjoy ir.
BaDaLing Great Wall and Ming Tombs day trip. Spend your money on a trip to the Great Wall at MuTianYu and buy some postcards of 'Spirit Way' (unless you like a 30 minute walk around an old basement).