Many wear ribbons around the neck as a 'fake collar'. You can also wear jewellry that is significant - i.e. 'this ring/bracelet/necklace is a sign of your submission'. Its bondage but not physical bondage. The concept of significant words or phrases to indicate what behaviour a sub should be following is another example of this - restriction of behaviour not movement. I suppose position training is another example - give a command, sub must stand/sit/kneel in a particular way. You can have quite subtle positions that are not obvious to onlookers but which the sub knows about.

Not bondage but how about intimate body jewellry with weights on or even body jewellry (nipple, labial or clit rings) which are linked together with chains or string? Possibly a bit extreme for many but you could, for example, 'lace up' the vagina by connecting a series of labial rings with string. Both of these could be under the clothes without anyone noticing. Similarly, nipple clamps could be on under clothes but you don;t want to leave them on too long...

As for vibrators... you may want to consider dildonics. These are vibes connected to a mobile phone which activate when you get a message. The intensity and frequency of the vibe is signalled by the content of the text message.