I like Phnom Penh (disclosure of bias: I lived there for almost 8 years) and think you can spend a couple days there easily, between sightseeing, shopping, eating and wandering around - though it depends on your interests, of course.
Three days is good for Siem Reap, as it allows you to mix temple visits with other sights, and include some downtime. I would avoid the boat from PP to SR (especially near the peak of dry season), and go by road (or fly) instead.
I also like Kep / Kampot more than Battambang. Finally, OP said they planned to fly SIN to PNH and return from REP.
With that in mind, I would consider something like this:
Day 1 - SIN-PNH (if I understand correctly, you're on jetstar, so I think you arrive in the morning, allowing time for seeing PP in the afternoon)
Day 1-2 - Phnom Penh (Tuol Sleng / Killing Fields, markets (Russian & Central) / higher-end shopping (e.g. Ambre), Mekong Cruise, Palace and National Museum, walking / cyclo tours, etc.)
Day 3 - to Kep / Kampot (around 3 hours drive; ideally via private car, if you want to do sightseeing / detours along the way)
Day 4-5 - Kep / Kampot
Day 6 Kep/Kampot to Siem Reap (full-day drive; avoid stopping in PP; lunch / rest breaks on the road around Snuol and/or Kampong Thom)
Day 7-9 Siem Reap
Day 10 - REP-SIN
I hope this helps!
Michael
Originally Posted by
jiejie
Here's my plan for you:
Day 1, SIN-REP flight
Days 2-4, Siem Reap. (Floating village in March dry season will be up on stilts rather than floating, ahem)
Day 5, morning bus to Battambang (3 hrs max), rest of day Battambang
Day 6, Battambang
Day 7, morning bus to Phnom Penh (5 hrs), rest of day PP
Day 8, PP
Day 9, PNH-SIN flight
If you had a couple of extra days, you could bus down to Kampot/Kep from PP then return. Don't cut a bus-flight connection too close though.
And wherever I use "bus" you can substitute "share taxi" or "private car" depending on your budget/time/convenience value matrix.