Time-Related:
* Late: late afternoon
* Early: early afternoon
* Golden: golden afternoon (suggests warmth and beauty)
* Sultry: sultry afternoon (suggests hot and humid)
* Pleasant: pleasant afternoon (suggests enjoyable)
* Lazy: lazy afternoon (suggests relaxed)
* Long: long afternoon (suggests it feels longer than usual)
* Short: short afternoon (suggests it feels shorter than usual)
Atmosphere-Related:
* Quiet: quiet afternoon
* Busy: busy afternoon
* Peaceful: peaceful afternoon
* Stormy: stormy afternoon
* Sunny: sunny afternoon
* Cloudy: cloudy afternoon
* Breezy: breezy afternoon
* Still: still afternoon
Other:
* Busy: busy afternoon (can also refer to a lot of activities)
* Productive: productive afternoon
* Relaxing: relaxing afternoon
* Enchanting: enchanting afternoon
* Memorable: memorable afternoon
* Romantic: romantic afternoon
* Serene: serene afternoon
* Dreamy: dreamy afternoon
The best adjective will depend on the specific context and the feeling you want to convey.