Like in many hill-stations, you'll come across an army of monkeys on the winding, ghat road. Pull aside to take pictures if you like, or slow down so that the kids can get a closer look at these lovely animals. But, don't get too close and don't provoke them, because they can get very, very nasty. Ensure (a) the windows are rolled up; and (b) ensure no food items are on the dash board, especially bananas. If they see a banana, you'll have a battle on your hands. Remember, out there you are pretty much out-numbered. It might also be interesting for you to know that a monkey bite can be pretty much fatal.
Whatever level of adventurous streak you have in you, don't overdo it by trying to crawl through bushes or thick shrubs. Yelagiri Hills is home to hundreds of snakes and some pretty venomous ones at that.
From Bangalore, get on to Hosur Road (NH7) and head towards Krishnagiri. The 4-lane National Highway is excellent and you should make it in less than 2 hours (crazy driving will get you elsewhere). At Krishnagiri, there's a toll-gate manned by the National Highways Authority. Pay your toll and at the second flyover (don't go over the flyover), take the service road towards the left. This road goes to Chennai. Take the left and for the next hour or so (approx. 50 kilometers), drive on NH46. You'll come to another toll-station. Pay and drive for a few minutes till you see Vanni Matriculation Higher Secondary School. Further up you'll see a sign 'Elagiri' (36 kms) indicated by a 'U' turn arrow (careful not to miss this sign) and that you need to keep left, take the 'U' turn, go under the bridge and on to the road towards Thirupathur. That's the road you need to get on to. Drive for about 7 kilometers (you'll see the winding road that leads up to the hills to your left). On the left, you'll see a big sign board 'Tamilnadu Tourism Department Welcomes You'. Turn left at this junction, there's a sign board pointing towards Yelagiri.
Fourteen hairpin bends and about half hour or so of driving uphill, you are in Athanur town. Hotel Hills is on the right side of that same main road (after the Yelagiri Hills police Station). You just can't miss it.
Take extreme caution while driving on the 4-lane expressways. It's not uncommon to see cyclists, two-wheelers or even cars and other vehicles coming towards you on the wrong side of the road.
Yelagiri Hills is also accessible by train. Many trains stop at Jolarpet and from there, one could hire a cab to reach Yelagiri. Haggle like hell and get the best deal. Frequent buses operate between Thirupathur and Jolarpet to Yelagiri.
You can get buses from Chennai, Salem, Hosur, Bangalore but a bus journey will eat into a major part of your day. Driving or taking the train is advisable.