Nowadays, everyone has a mobile phone and almost everyone needs to avoid being left with a flat battery at the end of the day. Give your customers a custom powerbank, i.e. a portable charger that is at their disposal when they need "energy".
The fact that powerbanks are used by everyone (young or old people, students or employees) makes it a very useful gadget and your customer will always have your logo in front of them when using them. This also applies to potential new customers you want to advertise to.