For first time international travel you need to choose the destination keeping in mind the following :
1 Budget
2 Least visa hassle
3 Ease of travel ( i.e direct flight)
4 and the weather My recommendations 1. Dubai
2 Singapore
You have many options in Asia.
If your family has a religious (Hindu) inclination, take them to Cambodia and show them the oldest and biggest hindu temple in the world. If there are old people and moving around is a concern, go for Singapore with all the modern facilities and it's very old people - friendly. Chiang Mai and Chiang Rai regions in North Thailand is also a beautiful option and so is A trip to the beaches of South Vietnam.
If this is their first trip outside the country then try to go somewhere else. To have them experience a wider change in culture and perspective than going to Sri Lanka.