In Siem Reap you should use USD. And in fact all over Cambodia. Amounts under one dollar is paid in Riel. 1$ = 4000 Riel. 25 cents = 1000 Riel. I try to always carry enough 1$ bills to to pay the Tuk Tuk the exact price, make a decent tip or to make the payment in small local shops.

ATM's are no problem in Siem Reap. In most ATM's you will be asked to choose between Reil and Dollars.

There are no cruising areas in Siem Reap or Siem Reap is a cruising area. I am sure your trained eyes will spot the potential wherever you go - and the potential is there. Maybe not in Pub Street as it is the main tourist "must walk" area.
Siem Reap will never be like Pattaya is/has been but more and more have discovered this way of earning a "little extra" within the last two years as more and more "farangs" are visiting. Go to Planetromeo and you will also see a lot of "potential".