During weekend, another good and cheap place to spend your time with your love one will be the airport. Why not think of going to the airport for a day out with your girlfriend?
In Singapore a trip down to Changi Airport by the Mass Rapid Train (MRT) is very convenient, if you have a car then it will be even better. However, just to a friendly reminder about the parking fee, it can cost you a bit if you intend to stay there for long.
At the airport, you can bring her to watch the planes taking off and landing, hold her hands and start drawing out a beautiful love journey, plan for country to go for honeymoon, it will be nice talking about some dreams and goals of the future.
There are also many shops and services around, if your girlfriend wants to do some shopping, she can do so as well, if possible catch a dinner over there will be nice before both of you heading back home.