I climbed Mt SiliSili in about 2001, so its a while ago and the players will have changed a lot, but here's a link to my write up from the time.https://iamfootlooseandfree.com/2016/05 ... sili-sili/
I did several other day walks on Upolu, but again too much time has passed, and pretty much any village will provide a guide to walk somewhere.
It is possible to walk into Lake Lanoto'o, then north down to the main road near Alesia. Long day going up, but there was a footpath sort of.
From Ti'avea on the NE corner east along the coast to Samusu.
From Salamumu on south coast through Paradise beach to Falease'ela
I also walked up Togitogiga Falls, and simply followed a footpath upriver until I ran out of time and turned around at a major unnamed waterfall.