My favourite short flights with default scenery:
KLAX to KSAN/KSFO/KLAS/KABQ (and return) - A320 / B727/B737/B767/B777
KSLC to KSFO/KSEA (and return) - A320 / B727/B737/B767/B777
KDEN to KASP in a Learjet, RJ
YSCB to YSSY (and return) in a B737, RJ
, Metroliner or SAAB 340.
EGLL to EHAM (and return) all aircraft
My favourites with added scenery:
YMML to YSSY/YPAD (and return) - A320 / B727/B737/B767
YSSY to YBBN (and return) - A320 / B727/B737/B747/B767
NZAA / NZWN / NZCH (all combo's) - B737 / ATR (B747 into ZZWN is fun)
RJAA to RJBB (and return) - all aircraft
EGLL to EGCC/LFPG (and return) - all aircraft
LIMC to LIPZ (and return) - B737/B727 / RJ
I haven't even touched on the US East Coast yet either
. I find the best thing to do is get an atlas (an old school atlas is fine) and look for major cities that are around 400-700 km appart.