Of Interest: downtown Busan, including the Busan Film Festival (October)
Lodging: not within sight, but if you walk to the main road there are some within 10 minutes. Ask at the tourist office
Tourist Office: yes. Also a good travel agency
Int'l ATM: There is a branch of the Busan Bank in the terminal
Restrooms: Clean and recently remodeled
I had a roundtrip ticket, but I missed the return date. I was STILL able to use the ticket. People often tell me that the plane is more expensive than the ferry, but discount ferry tickets are as little at 100,000. There regular tickets are good for six months and you don't even have to call if you miss your boat. (I suggest that people do call and cancel, or the company might change that policy.) They are almost the same price as a discount plane ticket which cannot be changed at all.
I took the Beetle/Mirejet high speed hydrofoil ferry that runs to Fukuoka. Kangsan Travel sells tickets and travel packages for this ferry.
There are also overnight ferries to Fukuoka, Shimonoseki, and Hiroshima. Most English speaking travel agencies do not sell tickets to these ferries, although there is a travel agent in the ferry terminal that does. If you can speak a little Korean, it is worth it to check out a regular Korean travel agency for tickets and ferry packages.
If you walk straightish along the road that leads away from the ferry terminal, you will run into a major thoroughfare. From there you have two choices. Turn left for one of Busan's downtown areas (there are more than one) or turn right for Busan Station and KTX trains. There is a subway station at the intersection. It is probably a good idea to get a map from the tourist office before you leave the ferry terminal.
The center of this downtown area is a semi-pedestrian shopping area.