I bought a three day pass when I visited because there is so much to see - that was 14 years ago though! The temples cover a very large area and some are really amazing. Ta Prohm is worth a visit on it's own (the jungle and the temple are all intertwined in a really cool way) but you'll need transportation between most of the temples.

I paid a moto taxi driver to drive me around and he'd wait while I explored the individual temples and locations - or I'd arrange to meet him again at a particular location. Rather than busily trying to explore every corner I took my time at each location and enjoyed the place. At Angkor Wat I befriended some local monks and they practised their English on me - in return I played them some music on my headphones! At one point I climbed a less popular but tall temple and sat there right at the top listening to music for about 45 minutes just having a great time enjoying the vistas. But I was travelling alone so other experiences will vary. Anyway the place is HUGE. Half a day gives you only a flavour of it all.

Definitey didn't regret the three day pass. I have better memories of those three days than the two or three days I would have spend in Siem Reap.

What would you do in Siem Reap on the days you weren't at the temples?

Halong Bay and Hanoi are great BTW. Enjoy!