I think Mikl's suggestion (to aid something while backed up by a toprope) is easily the best. A grade 18 climber could do 100 Araps routes below grade 10 and still be placing pathetic pro, because they aren't going to weight any of it. Weighting every piece on aid gives much more useful feedback - especially if they have to get creative when the bomber slots are more than 1.5m apart. Being on TR also lets them clean their own gear as they go, so they should also learn not to overcam etc.
I'd advise against Trapeze - on multiple occasions I've seen beginners facing a 15m groundfall when the Z-forces lifted out wires at the L end of the traverse. I'd also advise against all the multipitch suggestions, at least for starters! FWIW I've put new leaders on Exodus with no problems