Undercover Marketing is categorized under Guerrilla marketing. In undercover marketing buying decision are influenced without tipping off the consumer about the manipulation. Ad spies and leaners are actors hired to get people buying or discussing featured products. Also I can say that promotional products is quite powerful tool to improve your brand. Such items can be: personalised pens, coffee mugs, kraft pads, bottles, sports trophies, keychains, coasters, paperweights, customized t-shirts, shopping bags, fridge magnets, tea coasters, etc. You need not gift branded items like iPhone or play stations rather choose items that can be useful in everyday life. Many ideas for different budgets you can find here
http://www.conceptplus.ca/fr/