5 days in Singapore... not city people really!

If you are in Singapore I would recommend some of the museums in the city if you like that sort of thing, and the Singapore Zoo is of world class standards. Try China town and Little India areas for cheap food and accommodation. Theres also lots of outdoor sports on Sentosa Island.
If you do change your minds though, Vietnam is an amazing place. I know I would prefer 5 days in Vietnam than in Singapore anyday, especially if your on a budget!

Its useless to go to Brunei or Vietnam for just 1 day. Forget about Brunei anyway, its not interesting. If Borneo , the Malaysian territories (Sabah and Sarawak) would be a better choice.
Vietnam is a very interesting country. But the flying time back and forth alone will take 4 hours and more. Borneo is even farther.
For 1 or 2 days only the Malaysian east coast (islands or mainland) is a good choice from Singapore. Small and not very touristic places a lot.

I like Singapore quite a bit, but based on your desires I have a feeling that it is going to be a gigantic disappointment to you. Singapore has smallish beaches, but I doubt that they'll be anything like you want. Adventure sports? Well, near some of the restaurants that specialize in chili crabs you can go water skiing in a man made lake and be pulled by an overhead device, not a boat. I have no idea if that is adventurous enough for you or not. I did not make it to Sentosa, so I can't comment on that. A "tight budget" is also not a good idea in Singapore. I strongly suggest you re-think your plans.

Hey, you're not city people. Ok. But give Singapore a chance!

Don't bother with the beaches in Singapore, you're moving on to Bali...
As mentioned, Vietnam or Brunei for one day is just not an option, but you could do a short trip to Malaysia?
I'd just stay in Singapore and expore the city a bit, though.

The biggest highlight in Singapore is the food. So much, so good and so cheap (relatively). If you are a bit adventurous go to some of the more "heartland" food places (eg: Ghim Moh Market for breakfast). You get indian, chinese, malay, western and of course a mixed cuisine.

There's a couple of things you could consider if you are a bit into nature and relaxing for a few days:
- Botanical Gardens
- Sungei Buloh Wetland Reserve
- McRitchie Reservoir TreeTop Walk
- Pulau Ubin for some biking

And the museums (esp. ACM) are really good, too. And did I mention the Zoo and the Bird Park?

I'd stick it out in Singapore. I like Singapore much more than I thought I would. There are beaches on Sentosa Island. Like already suggested, eat out in China town and Little India.
I'd definitely go to the Night Safari and again, the museums. There is plenty to keep you occupied.

Besides the zoo, you can visit the Night Safari. You can combine as a one-day trip since they are located next to each other. At clarke quay, you have rides like the reverse bungee. East Coast Park (by the sea) is a good place to cycle and roller blade. In addition the few places mentioned by morid, you can consider Labrador Park with its WWII bunkers and quiet surroundings.

If you are the kind of adventurous and like those sport, Singapore is not a good place for you, more expensive and shopping center everywhere, I would recomment you to go to Malaysia, go to Cameron Highland or go to MT. KK in east malaysia, trekking or diving in Tioman Island also not bad

I spent a great day on Pulau Ubbin Island (north east), it's the only part of the country where people still live semi-traditional lives, there are are approx 100 people on the island and relatively few visitors. You can hire a taxi to take you to the ferry port, or I just asked my hotel which bus. The ferrys are regular. I rented a bike for the afternoon, very scenic and relaxing, definitely recommend it. There is a small info office and wildlfe walks and things can be arranged, but maybe in advance. I'm sure a guidebook or tourist office will have more info

Hi all

Thanks so much for all the info and advice, i really appreciate it. Is it difficult to get to Malaysia from Singapore? We are kind of tied to landing in Singapore airport but ive heard the islands around malaysia are amazing. Are they difficult to get to? Is it expensive?

Thanks again