Siem Reap is a pretty safe city, even at night. However, if you rent a bike, don’t keep your bag in the basket, as it will be easy pickings for a drive-by snatch. Likewise, lone females should try to walk home with travelling companions when leaving late-night spots, particularly if heading to poorly lit areas.
There are a lot of commission scams in Siem Reap that involve certain guesthouses and small hotels paying moto and taxi drivers to deliver guests. Ways to avoid the scam include booking ahead via the internet and arranging a pick-up, or sticking with a partner guesthouse if you are coming from Phnom Penh. Alternatively, just go with the flow and negotiate with the hotel or guesthouse on arrival.
There are a lot of beggars around town and some visitors quickly develop beggar fatigue. However, try to remember that with no social security network and no government support, life is pretty tough for the poorest of the poor in Cambodia. There is no need to give to everybody, but there is also no need to treat them as pariahs. In the case of children, it is often better not to encourage begging, but if you are compelled to help then offer food, as money usually ends up being passed on to someone else.
Out at the remote temple sites beyond Angkor, stick to clearly marked trails. There are still land mines at locations such as Phnom Kulen and Kbal Spean.