We only went to Apia for a day trip to do some shopping at the markets. We found the people lovely, the markets fair, and most things quite expensive.
Apia is very busy but has one major problem and that is there are no street signs anywhere. It must also have the only MacDonalds in the world that does not have WiFi.
Main town feels more like a village. Get your money changed, cos once you leave Apia it'll be hard to find a moneychanger or an ATM.
Amazing seafood. The tuna is out of this world. Spend no more than a day in town. Head out to the other bits - Lalomanu is a breathtaking, laid-back place to chill and soak in some Samoan hospitality, and splash around in some of the clearest, warmest pacific waters you'll ever see. Or head off to Savaii for more swimming, snorkeling, and close encounters with wildlife (dolphins! turtles! oh my!) I booked 4 days in Lalomanu. Totally insufficient! Two weeks would have been nicer.
Everything closes early so prepare. Bistro Tatau and The Office Café are great. Sunset on the harbour is a must.
Take a drive around the island. Be mindful of stray dogs at night otherwise very safe.
Don't miss the cultural show at the village behind the tourist/information building - it is free and amazing.
You even get a traditional lunch for free after you watch it being prepared and cooked! Catch the show with the fire dancers in the evening at one of several hotels - not something you see every day! Taxis are very cheap and most places close to the city centre or trips around town will be 5 Tala ($2.50Aus) and worth the money because the heat and humidity can be exhausting. I climbed Mt Vaea to see Robert Louis Stevenson's tomb but it is not for the unfit or faint-hearted - the tree roots and overgrown parts hazardous and where there are steps its steep. If you reach the top the view and sense of achievement are magnificent. RLS museum/house is not to be missed, the guides knowledgeable and obliging (ask them to sing the Requiem) the grounds are well kept and beautiful to walk around to learn about the plants and trees. You can hire a taxi for the day and be driven around the island and stop over the other side for lunch and also at a few of the waterfalls rather than hop on and off crowded local buses or hire a car. Always wear sunblock and carry water because the sun is strong and if you're not used to the heat slow down and take it easy like the Samoans until you are acclimatised - life is for enjoyment. There are heaps of churches everywhere but the catholic cathedral opposite the tourist bureau is well worth a visit.