In reply to Roger Burrows:
Can only recommend the walk in from the South/ East. Good for acclimatizing at around 3500 m. Going up to Eiger or Mönch from Grindelwald first thing is going to be hard.
You could take the bus up to the Grimsel pass, walk to Oberaarhornhütte, do Oberaarhorn before breakfast, return to the hut for breakfast, and hike across to Finsteraarhornhütte, then do one more summit from there on the way across to Jungfraujoch. It is of course possible to skip either of these huts, Grimsel pass to Jungfraujoch is easily doable in two days, one day only at push.
CB