Rather than attempt to create new greetings, we can look to other cultures to see which greetings they have which may be effective. For example, the French have turned their double cheek kiss, bises, into a verbal greeting, stating “bises” or the more familiar “bisous” to greet each other from a distance.
The namaste, wai or bow from many Asian countries are all examples of non-contact greetings. These gestures involve bending the body in a bow, and in the case of namaste and wai, holding the hands in a prayer position in front of the body. Interestingly, while the hands and arms protect the front of the body, they are still considered welcoming gestures.
And in several Islamic countries there has been a revival of the greeting which consists of placing the hand on the heart and bowing slightly.
Otherwise, if you want to keep it simple, a smile and wave, a head nod or a thumbs up are all effective ways to greet and acknowledge another person while keeping your distance.