I once had a large table that would have been great except for weak joints between the apron and table legs, which made the table wobbly. I wanted to strengthen the table without modifying it, and decided to see if I could retrofit mortise and tenon joints.

After removing the top, I drew in ¼” x 3” rectangles where I planned to add hardwood splines, at all 4 corners of the table.
First, I drilled ¼” x 2” deep starter holes at one end of the rectangles. Then I used my router, with a ¼” upcutting bit, to rout in several passes ¼” wide by 2” deep grooves along the length of the rectangles. Next, I glued and tapped in ¼” x 3” long x 2” deep hardwood splines.

The final result was a rock solid table, finally with strong joints.

This sketch shows the concept; looking down at the 2x2 leg and the 1x4 aprons that support the top. In dark is the added spline.

Step 1: Mark Out Location for Reinforcement

As you see in this photo, I marked with black felt pen where I will cut a groove with the router. The board held with the C clamp serves as a guide for the router. I have a 1/4" straight cut bit in the router.

Step 2: Cut the Groove

This photo shows the start of the cutting of the groove. You should take several passes with the router, extending the bit about 1/4" deeper each time. You should end up with a groove at least 1 1/2" deep.

Step 3: Groove Completed

This photo shows the completed groove (first if 4!) between the table leg 2x2 and the support apron rail 1x4.

The grooves were about 2" deep.

Step 4: Glue and Tap in the Spline

Here goes one of the hardwood splines. I'm just reinforcing in the long diminsion.
The grain on the spline should be legthwise, that is, from left to right in the photo.