There is a very nice guest harbour centrally located in Arendal. Fees: NOK 200 / 250, penalty NOK 500.
At the reception area there are showers and toilets, beach and a restaurant which once in a while arranges a dance.
You switch trains at Nelaug, the local train is waiting whenever you come. The local train used to traffic the line south of Stavanger, and before that the Oslo area. A bit nostalgic to meet it again :-) (Yes, there is only one.)
This is the town square, with the subterranean parking house in the background. The big secret is that by entering the parking house you can walk or cycle through the mountain and right to the railway station :-) If you go by car, you have to drive around, as the road leads to the parking area only, but the sidewalk goes all the way through.
You can get there by local train from Nelaug. When you get of the train, walk behind the station building, that is straight ahead, cross the parking lot, go up the small road and turn immediately right into the spooky looking tunnel, which will lead you past a subterranean parking house and right into downtown Arendal.
Out on Tyholmen out along Pollen's right side, there is the ferry quay for Tromøy. Tromøy is a suburban island but also big enough for a bunch of other interesting things tro see and do - mainly seaside wilderness.
You can hire a boat as a taxi to take you where you want to go. Some bigger boats will also happily do sightsseing trips and fishing tours, picnics, group tours and what have you. Down in Pollen bay you find a couple fo boats/berths advertising for this service.
There are several islands around Arendal that are inhabited and considered as suburbs, more or less. Short ferry rides go to these islands, plus there are some bridges so buses are an option, too. It takes some time to get your directions right, but nicely, maps and instructions are put up at departure points. Also the tourist info will be able to tell you where to go and what to do out on the islands.
Buses tend to reach Arendal faster than trains. The Oslo route takes about 1 hr less time, in fact. Arendal is well connected up and down the south coast and inland, too, by bus.
The bus station is located in the centre of town - basically it's the two streets one on each side of the muncipal offices/house of Culture building. The bus time tables/departure times are flashing electronic things put up above the sidelwalks. Easy to see from which numbered bus field and at what time they depart.
A very quaint train station at the end of the line - ligterally. The locomotive sheds have been turned into shops and cafes. Goods and bulk cargo transportation has come to an end, too.
But the station is in operation nevertheless, with opening hours 9-16:45 on weekdays, a bit shorter on weekends. A note on the door says that if the station is closed you can get your tickets on the train. There is a waiting room and toilets but no other facilites.
Arendal is at the dead end of a branch line of the railways. To reach Arendal you will need to travel on the Sørlandsbanen and get off at Nelaug. From Nelaug you jump on the two-coach suburban type train that ply the half-hour trip to Arendal. Very pleasant view of settlements, lakes and rivers on this stretch, plus some ugly hydro power plants and dry river beds. There are not that many connections to Arendal, so consult the train table before you go.
Arendal is about mid-way between Oslo and Stavanger. Train travel time is about 5 1/2 hrs by train, faster by bus from Oslo, but less comfortable.