Not taking expenses into account. Purely based on what there is to do/see.
I've never been to Fiji but I really enjoyed my stay in Langkawi. It's such a small island that if you rent a car, you could go see a lot of places in one day, gas/petrol is very cheap too! (2RM per liter).
Sharing my itinerary...
Day 1 Arrival in Langkawi
Day 2 Langkawi Cable Car (this is situated in Gunung Mat Cincang) / Seven Wells Waterfalls (while on our way we saw a lot of macaques and a group of Dusky Leaf monkeys)
Day 3 Pantai Cenang/ Cenang beach (The beach is not so breathtaking for me but lots of choices for restaurants and souvenirs. You can also do paragliding and jet ski island hopping tour in this area.)
Day 4 Island hopping tour (visit Lake of Pregnant Maiden, Singa Besar Island, & Beras Basah Island)
Day 5 Visit Pulau Payar Marine Park (really good for snorkelling! blacktip reef sharks are everywhere)
Day 6 drive to Tanjung Rhu beach
Day 7 Mangrove tour (Kilim Geoforest Park)
Day 8 drive to Pasir Tengkorak (my favorite beach in Langkawi!)
My itinerary is a bit chill but you can definitely shorten it. There are also lots of other things that you could do like visit zoos, aquariums, cultural places, agricultural farm, visit Koh Lipe,Thailand etc. You can download Naturally Langkawi's app to get a feel of it.
Hope this helps! Cheers! almost 5 years ago