On Upolu, taxis can be a useful transport option for day tripping; however, the same can’t be said for taxis on Savai’i, which are only convenient for short trips. It always pays to have the correct change as drivers can be (perhaps too conveniently) relied upon not to have any.
If you find a driver you hit it off well with early on, it can be worth getting their telephone number and using their service during your stay. You may be able to negotiate a decent day rate that compares favourably with a hire car.
Local buses are cheap and run all over the islands frequently (apart from on Sundays).