We almost didn't go to Sepilok, put off by its association with mass tourism at the Orangutan Rehabilitation Centre.
But we had a few days to fill in and further reading suggested other possibilities for the area.

We weren't disappointed; to the contrary we regard it as one of the highlights of our time in Sabah.
There are two lesser-known features of the area that we strongly recommend:
* the Sunbear Rehabilitiation Centre; and
* the Rainforest Discovery Centre.
The Rainforest Discovery Centre embraces a substantial area of primary lowland dipterocarp forest with a maze of walking trails and an impressive set of canopy towers and a canopy walkway.
It is a much under-appreciated resource and a wonderful place to see birds and other wildlife.

And the Orangutan Rehabilitation Centre had its positive moments too.
One was the opportunity to get up close to (and photograph) free-living Orangutans, though they are fed and thoroughly used to people. (We did see them in the wild at two subsequent locations.)
The other was the feed station. We stayed long after the madding crowd had departed, perhaps for an hour, and were treated to a succession of species coming in, Orangutan, Pig-tailed Macaque, Long-tailed Macaque, Prevost's Squirrel and even a bird or two.