Advice on Cambodia?

I have will do a visa run and have a flight to Phom Penh on June 30. Never been to Cambodia before.

Will do some research on internet (including reading recent reports on this and other forums).

I have got some information from a friend. The data below is what he told me, please correct or add if you know better (my preferences: public transport, low cost accommodation, food and entertainment).

I will get visa on arrival at the airport (20 USD, and only USD will be accepted). Pay in USD, get change in Riel. Should I change Baht to USD in Thailand or withdraw USD at an ATM in Cambodia from my Euro visa card? (Probably the latter, EUR -> THB -> USD has two conversions, EUR -> USD only one.)

I will spend a few days in Phom Penh. There is a market hall in Art Deco style, I want to see it. Places related to Red Khmer interesting as well. And once having seen this market hall and being in Cambodia anyway, I can as well travel to Siem Reap (bus, 6 USD) to see Angkor Wat (30 minutes by bicycle, entry 20 USD). Any other places (I like the weird and unique, however in Cambodia I canтАЩt travel as freely as in Thailand because I donтАЩt speak the language.) I wonтАЩt go to Sihanoukville, a friend is going to move there, so I will wait until he is there to visit him. I am not interested in beaches (unless there are cute boys in swimwear), but I like mountains and forest.

Gay activity not important, but after dark there wonтАЩt be much else to do, so I can as well have a look at gay discos, massage, bars, cruising areas. I probably wonтАЩt use grindr or gayromeo. Any suggestions? Differences to Thailand?

Return to Thailand by bus (or better book a flight from Siem Reap, new visa regulations for land borders?)

Where to stay in Phnom Penh and Siem Reap? (I read the River Queen Guesthouse blog, so I might stay there in Siem Reap.) No need for gay places, location (to minimize travel time) and price is more important. However joiner friendly would be advantageous, you never know.

Why Cambodia? I have been to Lao twice, now itтАЩs time for something new. When I applied for tourist visa a the Thai embassy in Berlin, I was asked to provide proof of leaving Thailand, which caused some inconvenience. So I booked a cheap flight on Air Asia and went back to the embassy the following day.

Booking a flight with Air Asia is a story for itself. The flight was priced at 1590 Baht, I got away with 1690
Baht, but their booking system is tricky, there are options that cost more everywhere, and how to book without these options is in the small print. Hold luggage costs extra, I have only cabin luggage which limits what I can take with me for the trip.