Jerry,
    That's exactly what I was thinking yesterday.  Although finding
geocaches is obsessive enough of a hobby, I don't want to start
challenging myself to see how many travellers I can find.
    For the past week I've been toying with the idea of making a
permanent Traveller cache.  I'd start the cache with a logbook (of
course) and 3 travellers (I would get from other caches).  Then each
person, to log a successful visit, would need to bring another Traveller
and exchange it for one of the Travellers in the cache.  Of course I'd
need to be sure if the geocaching.com website would work tracking 3
travellers in a single cache.  The person visiting would have the
challenge of nabbing a traveller before he/she could visit, then have
the choice of which traveller to take (depending upon each traveller's
goals).
    I think such a cache would work for awhile, but over time I think
the travellers would disappear and non-traveller items would eventually
fill the cache.  With my White Elephant Cache having specific rules,
I've found 1 or 2 visitors have totally ignored the rules and treated it
like a normal cache.  And one person in the log even complained that the
cache had too many rules.  (It has I think 4.)  I'm sure at some point
I'll need to go to this cache, clean it out and restart it with a single
White Elephant gift.
    Well, my mind has been cranking thinking of unique cache ideas.
That's one of them.