Are you going to be based in Apia?
If so your main options are catching the bus-which is only really suitable if you want to spend the night somewhere, or taxi. The bus is cheap but time consuming, maximum cost down to the south coast beaches is about 10 wst but the schedule is geared toward the villagers coming into Apia for the day so not that easy to do a day return the other way. To use the bus decide what beach you want to go to then ask the bus occupants at the bus depot for the appropriate bus and time eg for Taufua Fales take the Lalomanu bus, for Faofao & Manusina take the SALEAPAGA bus, for Vaiula fales take the Tafatafa bus. The buses leave about 9ish, 11ish - a bit flexible within about an hour or so and take about two hours to get there with many stops along the way.- so use most of you morning getting to one beach.
Taxis are plentiful and not too expensive- around 80 tala or so down to the coast from Apia and most drivers will offer a tour for the day for a few hundred or so.
The only bike hire is out by the airport and the ferry to SAVAII - check out outdoor samoa or bike Samoa website.
Biking is a good option on SAVAII where there aren't so many big hills but quite an effort around Upolu- and I found the traffic and dogs worse on Upolu too.
There is a guy renting motor scooters on Savaii now, at Asaga, - 50 wst a day, he owns a bakery / store / coffee shop there, he's on facebook too just can't remember the name.
If you don't need to go to Apia and you just have 5 or 6 days you may want to go straight from the airport to Savaii. The buses are a little more straight for ward, they meet each ferry and head to their village, just get off the ferry when all the locals do ( normally queue before docking) and ask someone. The bike and scooter options work better there also, Savaii is a bit further from the only city ( Apia) so is a bit more authentic, a bit cheaper, a few good beaches- Lano, Manase...
The island is about 250 kms around so 3 or 4 days is nice to explore and then maybe a day or so in Apia before you fly out
