1) travelling HLO, how much before the flight would I need to get to the airport
I'd say leave about 2 hours prior to departure would be ideal. 1.5 hours is safe. The only area where you might lose time is the security. If there is a queue, then it can be delayed a bit, but otherwise MAA domestic is easy to navigate. Check-in closes 1 hour prior to departure, so better to check-in online (given you are HLO).
a) how reliable the trains are
If you are talking about Metro to the airport - then I don't have much to offer in terms of info. I have never used the Metro so far. I usually have my driver take me from/to home/airport. Or I take a call-taxi (my parents always use FastTrack call taxi). There is an airport station for the Metro line.
Note: Call Taxi charges from the airport to outside is expensive compared to call taxi from outside to the airport. They are usually outside arrivals area in small kiosks. Depending on how far you like to go and the type of car you choose, you could be paying anywhere between $10 to $25.
But I have been told that Metro is very reliable and easy to use. So depending on where your work place is, Metro might be your ticket. If you are working anywhere near the Olympia Tech Park area, then Metro is your best bet (it is closer to the airport, you can actually see planes landing approach from Olympia)
If you are talking about EMU trains (Southern Railways trains) - then you will need to get down at Thrisulam train station and walk across the road to the airport.
b) what to expect on a local train.
You will need to purchase ticket beforehand. Regular train service, nothing fancy.
I'll only need a hotel for a night. Where should I stay?
Depending on where your workplace is - stay closer I'd suggest. Roads can get very busy during rush hours (say, 0700-1000 and 1600-2000). There are some really good hotels near the airport area (say, 10kms from airport). There is a Hilton near Guindy which is good.
And if I have time for one tourist activity/thing to see, what should it be?
Depending on what you like and how much time you have, you can visit Mahabalipuram. It is approx 2 hours by road and worth a visit.
Drenched in an enigmatic old-age charm, Mahabalipuram will amaze you with its rich history, thriving present and prospects of a glorious future. Noted in history as a busy port town, Mahabalipuram rose in stature and importance during the period between 4th and 9th century when the Pallava...
www.tamilnadutourism.tn.gov.in
On the way to/from Mahabalipuram, you can also visit VGP Marine Kingdom. It is the largest marine park in India
VGP Marine Kingdom is India's first underwater aquarium housing small to big aquatic animals, housed in life-size tanks and over-head aquariums. Book Now!
vgpmarinekingdom.in