Phnom Penh has regular flights to other major cities in Cambodia (Sihanouk, Siem Reap) that leave multiple times per day are not scarily expensive.
There are international flights to major Asian cities from Phnom Penh, including Vientiane (Loas) and Hong Kong (where you can get direct flights to the US).
You can also fly from Phnom Penh to Kuala Lumpur (Malaysia). KL is is Air Asia’s hub and makes most of Asia very accessible and cheap. For example, once in KL you can fly to Bali (DPS) for under $30 USD.
Getting to Phnom Penh from the Airport
By taxi: You’ll pay about $15 USD for the 30-minute ride to the city center.
By Tuk Tuk: Expect to pay about $10 USD for a 45-minute ride.
Buses from Phnom Penh to other Cities
Buying your ticket from your hotel is the easiest way to purchase a bus ticket. They can arrange a bus or minivan ticket. Minivans may pick you up right at your hotel, while you’ll need to head to the bus station to catch intercity buses.
Grab a TukTuk and head to the bus station which is centrally located to pick up your bus. The TukTuk should be about $3-4 USD from any location in central Phnom Penh. There are restrooms, a coffee shop and convenience store at the bus station.