Hi guys,
1.) Where to get a car rental in Venice? We are planning on staying in Venice for 3 days. However, the middle day, we were thinking of doing a day trip to the Dolomites to go for a hike. When I was looking at reviews of car rentals around Venice, they didn't seem good.

2.) If we are going to visit the Dolomites for a day, which hike is best?

THank you

That would be too much time spent round trip(5-6 hours?) for me to do a day trip to the Dolomites . Add several days in the Dolomites before or after your time in Venice. Three days should all be spent in Venice alone.
. Car rentals can be found at Piazzale Roma Station.

I wish we could stay more. Definitely will make a trip just for the Dolomites next time we go to Italy.

Is that three days/four nights for Venice, or two days/three nights? Either way, too little time for Venice itself, there is so much to see there. Ditto the Dolomites. If you want exercise you can get plenty walking around Venice. BTW, you can see the mountains from Burano.

The advice so far is pretty good, I can't improve on it. Yes, spend the entire three days in Venice and do your mountain hike another time.
A number of years ago (2012) my wife and I began a trip in Venice, rented a car in the Piazzale Roma (there are several option), and proceeded to Bolzano in the Dolomites. The logistics of getting from your hotel in Venice to the Piazzale, picking up a car there, and then reversing all that makes the round trip travel time into the Dolomites more like 7 to 8 hours.