Achimota Forest is one of Accra's rare large green lungs, a remnant forest reserve hemmed in by the city's northward growth. It is not a manicured botanical garden, and that is part of its interest: paths, scrub, woodland, and birdlife survive here in a capital that is rapidly eating its open space.
Go in daylight, preferably with someone local who knows current access points and conditions. It is most rewarding if you want a break from traffic and concrete rather than a polished attraction with signs and facilities.