For the flights Bkk-Siem Reap-Phnom Penh-Bkk, we went with Bangkok Airways.
I can't remember the price we paid (somewhere in the 200+ USD per person) but it was cheaper to book all flights with their "Discovery Airpass" option than any other option available at the time. Under this system you book between 3 and 6 flight sectors....the international sectors (Bkk to either Siem Reap or Phnom Penh and return = 2 sectors) and then book one domestic sector between SR & PP - total 3 flight sectors.
There are other options shown on the website - multi-destination and their promotions.
I've just had a look at their website and they have a '2 for 1" promotion on from June-Sept if you book with American Express card.
You'd need to go to the website - bangkokairdotcom - and check prices for these options to see which one works out cheapest and most convenient for you.

Otherwise you could book Bkk-Ho Chi Minh -Bkk with either Bangkok Airways or Air Asia and leave yourself enough time to do some domestic flights within Vietnam or else a cheap flight HCM- Cambodia once you get there.

The must sees in Siem Reap -
1. of course the temples, but we found seeing two was quite enough seeing we only had four days there. How people can go 'temple viewing' every day is beyond me.... very tiring in the heat.
We chose Angkor Wat and the one that was used in the film Lara Croft Tomb Raider movie - that's the temple that's slowly being consumed by the huge tree roots... quite amazing in places. We just got a tuktuk to take us and wait outside.
2. night market (a very interesting open air bar right at the back too with a bar tender who does the bottle juggling thing)
3. Pub Street (lots of little bars and restaurants)
4. Cultural Centre out on the airport road.... we didn't see this either but sounded interesting enough.
5. A trip in a tuk tuk is an interesting way to see the place.... the main road near the local market a real eye opener... amazing what you see in the traffic....... e.g. motorbike with 3 pigs splayed out across the pillion seat ! then our driver did a uturn in the middle of a sea of motorbikes. YIKES!
He also took us around the back streets where we saw a stone carving workshop (sweatshop) and lots of shanties on the river - such poverty but such friendly people - the little kids came out to wave hello and were delighted when we took their photo and showed them.
6. A visit to the Foreign Correspondents' Club - this has been turned into a very stylish restaurant/bar with upstairs dining room and balcony - rather Raffles style with the colonial architecture and the overhead fans. There are lots of framed photos and newspaper clippings of the Khmer Rouge/Pol Pot era.

In Phnom Penh the 'must sees' are a bit more depressing.....
1. Tuol Sleng - the high school that was turned into a prison camp/torture centre. It's a sobering experience like many such historical sites but gives an understanding of the events that have shaped this country.
2. Killing Fields - I didn't go there (one sobering experience was enough for me) but my husband went and he said it looked just like a paddock but it was the knowing what it had been that made it sombre.
3. The Royal Palace/Silver Pagoda and other buildings in the compound... this is in town on Sisowath Quay, so it's easy to get to from most of the hotels in that area.
4. Lots of interesting bars, restaurants etc. along Sisowath Quay.
5. The Russian Market - quite big and you can buy everything from silk by the metre, items made of silk, wooden articles etc.etc. as well as shoes, clothing, jewellery and other stuff that tourists seem to want.

Both Siem Reap and Phnom Penh cater well for tourists but we liked Siem Reap better because it was much more laid back and easier to get around.

For more information on both of these places - photos too - suggest you take a look at Cathy and Gary's travel pages........... the link is on here somewhere. Mark????

If you feel like a bit of a beach break, you could do that in Thailand or Vietnam easily enough. I'll be posting a JBR Part 2 on here later this week.