What would be included in a thank you letter to Boo Radley if Jem, as an adult, were to write one in the future?

If Jem, as an adult, were to write Boo Radley a thank you letter, he would thank him for leaving the gifts in the tree, for mending his pants, and for saving his life on the night Bob Ewell attacked him. 

In chapter 4, Scout finds gum in the knothole of the Radley tree. A little while later, Scout and Jem find two "Indian-head" pennies: one dated 1906 and the other 1900. The kids consider these good omens and can't wait for what they'll find next. A few months later, and in chapter 7, they find the following:

". . . two small images carved in soap. One was the figure of a boy, the other wore a crude dress . . . Our biggest prize appeared four days later. It was a pocket watch that wouldn't run, on a chain with an aluminum knife" (59, 60).

After this big haul, Jem thinks about writing a thank you note, but Mr. Nathan Radley discovers what's happening and fills up the knothole with cement.

Between finding things in the tree, Jem finds himself in a predicament on the last night of summer vacation. In chapter 6, Jem, Dill and Scout go to the Radley house at night to get a glimpse of Boo through a window with a broken shutter. While escaping Nathan Radley's gunshots, Jem's pants get caught and torn in the fence. When he goes back for them, he finds the pants crudely mended and folded, waiting for him to come get them. This saves Jem from getting in trouble with Atticus, so he would thank Boo Radley for helping him that night as well. 

Finally, and probably most importantly, Jem would take the time to thank Boo Radley for saving his life on the night of the Halloween festival at the school. In chapter 28, as he and Scout are walking home that night, Bob Ewell jumps the kids with a large kitchen knife in hand. Jem's arm is broken in the scuffle, but it is Boo Radley who defends the children and carries Jem home. If Boo had not jumped in to help the children, they probably would have died.

If Jem, as an adult, wrote a thank you letter to Boo Radley, he would include all of the above events in his letter. He also would not call him "Boo." Out of respect, he would start his letter with "Dear Mr. Arthur Radley" and then mention all of the above items as listed.