Unfortunately, there is no free access to the Sky Bar at Marina Bay Sands. It's part of Ce La Vi, so you would be expected to get a drink or a bite if you want to spend time there! If you're a guest at the hotel, you'll be able to access the hotel's famous infinity pool which offers you equally breathtaking views. If not, I would suggest having a drink at 1 of the rooftop restaurants (instead of paying for the Observation Deck) as you can enjoy a drink AND the view for the same price! My personal favorite MBS restaurant is Spago. (The food and service at Lavo are both not good. However, if you just want to enjoy the view, the view there is great. I've visited the Observation Deck, Spago and Lavo, so my recommendation is based on personal experience!)