Just Returned from 3N4D in Siem Reap only. Honestly i don't think enough time to do Siem Reap and Sihanoukville.
Stayed at the Sarai Resort and Spa, great location (1km from pub street and surrounds) and absolute fantastic hotel (maybe a bit pricey). Booked all activities through the hotel as they just contact the relevant outsourced guys.
I would say no need for travel agent. Did the following:
Quad Biking - Half day tour, drove up to Tonle Sap lake and took a little boat ride (Boat ride = USD $20 pp), it was cool, even let me wife drive the boat, just don't get caught in the tourist traps of crocodile farms, charity to kids (ok this helps, but its expensive.. $50 for a bag of rice). Quad bike ride itself was AWESOME, so much fun.
Phare Circus - Super energetic little circus performance from a school that helps under-privileged kids, good fun, just don't go expecting cirque du soleil. Nice evening out, enjoyed this.
Rosanna Theatre - It was okay i guess. theatrical performance about each asean nation, some awesome costumes, little out of sync, but good for a night out if you have nothing else to do.
Sunrise Angkor temple tour - we used the car option from the hotel because its REALLY nice to get into a car with AC and the driver hands you a cold towel to wash off dust and cool you down. Sunrise tour included the 3 main temples - Angkor watt, Ta Prom and Bayon.. honestly thats moreeeeeeee than enough temple unless you really love them (but gets a bit much after this). 4:30am - 11:30am
Cooking class at Lilys secret garden - the guy takes you to a local market, buys some ingredients, explains a lot about the food/harvesting and how markets work. We learnt to make Red Curry, Mango salad, pumpkin in coconut milk. also quite cool.
Randomly when we were looking at Wat Preah Prom, a tuktuk driver came and offered to take us to Royal Residence, some other temple, war memorial... very average imo, takes you to indian shops where he gets entered into lucky draws for bringing customers... not recommended.
Food - LOVE it, so nice. Amok Curry, Beef Lok Lak, Mango Salad, Bannana flower salad... super good stuff.
Markets - Avoid the central market.. go to ones like Made in Cambodia (Behind hard rock cafe)
Be aware though, it may look and sound cheap but everything adds up very very quickly. the ONLY cheap things are : Beer (.50c/$1), Massage ($6-$20), Cigarettes ($1.70). Everything else is not cheap, roughly $3-7 per meal per person. $2 tuktuk, $37 angkor entrance PP and then Tipping. All in USD!
Overall it was a super enjoyable trip, one of my favourites and soo much fun. Cambodian people are super friendly, especially at the Sarai hotel, wow!