If you find 3 days are not enough for you then take 1 day more to discover, I don't have many leave left so it very depends on destinations.
I can share you some of my trips over long weekend as :
1 - Hochiminh & small cities around.
2 - Danang, Hoi An
3- Hanoi + Halong Bay
4 - Krabi
5 - Phuket
6 - Bali
7 - Brunei (quite boring actually but I bought a tour to forrest then become quite nice in the end), 3 days are more than enough.
8 - Darwin
My next trips over long weekend will be : Penang + Langkawi, Chiang Mai, ...
When you plan to travel in budget airlines, you need to book very early to get the good deal. But many people want to get out of Singapore over long weekend so asap is always better

The reason I often travel to these places because they have budget airlines going there and I like backpacking

Hope this is useful for you guys.