There are several tracks to choose from around Lake Alexandrina and Lake McGregor. Lake Alexandrina is made up of a few dozen holiday homes, as well as a camping ground. There is a walk which takes you along the lake shore from the campground on the southern end of the lake.
However, the walk that gives you the best view of Lake Alexandrina follows the walk up the 4WD track near the car park. From this viewpoint, make your way across the little bridge to the right and walk along the lakeshore next to the little holiday homes. It is impossible to get lost, so follow the walking tracks wherever they may lead.
As for Lake McGregor (which is right next to Lake Alexandrina), it also has a nice lakeside walk. Luckily the tracks are elevated so you will be able to enjoy the views from above. Lake McGregor also has a few partially submerged trees near the shoreline which are an interesting feature.
In the summer, you will be able to experience the lupin flowers that (though technically an invasive species) the area is known for. In the winter, experience the awe of the snow-covered mountains beyond Lake Tekapo.