(1)Jungfrau is expensive, so waiting for good weather is important--but on a 7 day trip could be difficult.
(2)In May the days are longer so you have more time to see the spectacular scenery.
(3)The "named" trains require a reservation and a supplement.
(4)Luzern is less touristy than Interlaken, less hectic than Zurich.
(5)A few trips you might investigate:
A. Luzern to Vitznau (by boat), cog railway to Rigi Kulm (spectacular scenery); Rigi to Arth Goldau, train to Goschenen (trip spirals around the Wassen Church) [When the NEW Gotthard Tunnel opens In June 2016, this trip may be difficult to do], Goschenen to Andermatt then to Visp to Zermat (overnight stop(?) somewhere in between)
B. Luzern to Verkerhaus Transport Museum (1/2 a day), Voralpine Express trip.
C. Luzern to Interlaken (via Meiringen) super scenery; either Jungfarau OR on to Spiez and Zweisimmen and then to Montreux.
There is NO end to scenic trips...have FUN.