14-16 day travel to Thailand and Singapore

Singapore is located only about 50 miles north of the equator so there aren't really seasons - just the dry and the wet times of the year known as the monsoons. Suggest that you Google "Singapore monsoon season" and study up on it before you commit, but generally Feb thru April are the best times of the year to visit. It'll always be hot and humid during the day (mid 90's every day) and doesn't get much below 80 at night. Thailand is much the same - count on it being hot and humid all the time.
Singapore is the cleanest and safest travel destination in Asia if not the world. The whole place runs as smoothly as a Swiss watch. There's a lot to see and do, especially if you're at all interested in British Colonial and WWII history. Your best bet is to get a good guidebook (or three) and start your research. The "things to do" pull down menu on Trip Advisor can be helpful too. Depending upon your specific interests there's plenty there to keep you occupied for a week.
Same with Thailand. Time and interest permitting you might consider a stop along the way in Malaysia. Kuala Lumpur is an interesting place and the Malaysian countryside provides an interesting contrast to the city-state of Singapore. Probably the only place you're likely to see temperatures in the 70's would be in the Cameron Highlands region - the Brits established tea plantations there back in the colonial area and it's still one of the unappreciated (and as yet undiscovered) little gems of the region - worth a side trip anyway.
Suggest extending you trip in the region for as long as you can - it's going to take you a full day of travel each way to get there. Might as well make the most of it while you're at it.
Another fascinating place to visit, maybe as an enroute stop on your way to or from Singapore, is Hong Kong. Booking a multi-city itinerary would allow you to maximize your available time without having to backtrack for your return flight.
And if you have the option to do so, by all means book your flights on Singapore Air - probably the best airline on planet earth.
Safe travels.

Okay, I've picked myself up off the floor (laughing) after reading "I dont mind if weather is on the cool side (70s)" As Robert said, cool doesn't happen there.

The best time - aka the dry season - is December-February in Thailand. I think of the country as 3 areas: the north, Bangkok, and the south. While there are some sights to see in the south, the big attraction is the water, beautiful sandy beaches, snorkeling with the tropical fish and colorful corals, and relaxing. The north is mountainous, so it is generally less humid and while it doesn't seem to cool off much in the evenings, the mornings can be very pleasant, even a tad chilly. The two main places are Chiang Mai and Chiang Rai. Bangkok is hot and humid. For a little relief, you can take taxis, which aren't expensive and the AC is always blasting. I like to stay in a non-touristy area close to the river and take the river boats to sights. It's always cooler on the water.

Plan on using planes to fly from place to place. Trains and buses are substandard and slow. Flights are cheap and frequent.

One very popular side trip is to Siem Reap for 2-3 days to see Angkor Wat and the other temples.

I spent very little time in Singapore and it was enough. It is very expensive. To me it felt like a super Disneyland, too perfect and a lot is artificial. The part I really enjoyed was Little India - much more like Thailand or Vietnam than pristine Singapore.

singapore accomodation can be expensive, its a nice city to look around, the taxis are plentiful and not overly expensive. we stay in an apartment style 'hotel' ( to keep the cost down a bit) that also includes breakfast, it is close to most things in the city. so do your research. food options are plentiful. the garden by the bay park area is very nice
the airport is nice and an easy trip into the city.
hope this helps.

I'm laughing now too. Just checked the weather averages for these places and there def. is no cool weather! I guess I would like to see a couple interesting cities in Thailand - prob not the northern side though. Mostly instead in beaches. Thanks for your replies.