Rensselaer Lake, also known as Six Mile Waterworks, is probably one of the best parks in the Albany / Capital area that many people (including local people) don't know about. You can reach Rensselaer Lake off the roundabout off the strip of Fuller Road that runs between Central Avenue and Washington Avenue.
There is generally plenty of free parking, picnic areas and a small playground. The most popular activities at the park tend to be hiking (there are miles of trails that will lead you into the Albany Pine Bush area), fishing and boating (kayaks and canoes - no gas motors permitted on the lake). It is possible to hike from Rensselaer Lake through the Pine Bush to Cook Park in the Village of Colonie; however, this will require crossing some railroad tracks which could raise both legal and safety issues depending on where you cross.
In addition to the main lake area, boaters are able to navigate a very narrow and shallow strip of water that flows below the Northway (Interstate 87) and which then opens to two additional strips of water which can be explored. I have pasted a picture below taken from Google Maps which depicts this. Tip: The L.LBean store in the Colonie Mall offers "adventure" kayaking outings starting at around $20. This is a great way to "sample" both kayaking and the lake without committing to all the expense and overhead of purchasing and hauling equipment.