If you're looking for a meeting place or a good stop around ½ or halfway between Mechanicsville and Gainesville, VA, you can try Massaponax, VA, which is almost an hour north of Mechanicsville, or almost an hour south of Gainesville, VA.
Other possible places to stop near the midpoint include Cottage Green, Leavells, and Confederate Ridge.
About ¼ or a fourth of the way, you can stop at Caroline Pines, VA. This is about half an hour north of Mechanicsville.
About ⅓ or a third of the way, you can stop at Ladysmith, VA. This is about half an hour north of Mechanicsville.
About ⅔ or two thirds of the way, you can stop at Stafford, VA. This is about 1 hour north of Mechanicsville, or about half an hour southeast of Gainesville, VA.
About ¾ or three fourths of the way, you can stop at Montclair, VA. This is a little over 1 hour north of Mechanicsville, or about half an hour southeast of Gainesville, VA.
If you drive straight through without stopping at all, the total distance is 99 miles. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 1 hour, 40 minutes.