There is a free shuttle bus to La Ronde from the Ile Ste. Use of the metro will probably take 45 - 60 minutes maximum in each direction, but a car is definitely more direct. You should be able to find free overnight street parking near the hotel. Just remember to remove all items visible to possible thieves. Montreal is a very safe city if you use street sense, but we're known as the car theft capital.
Trains run very frequently.