This scavenger hunt is getting boring. Let's go work on the treehouse!
Cueball and Megan are seen running together hand in hand. Behind them, two others (who look like Cueball and Megan as well) are standing next to a box in an open field of grass.
Together, Cueball and Megan fall in love, get married, and retire in old age to a porch swing, Cueball now wearing a sailor cap and Megan with hair worn just like Hairbun.
Suddenly, without a word, Old Cueball using his cane, leaves old Megan on the porch, even though she shouts out for him to come back. He then returns to his two friends at the box from the first panel, they are now also much older and are still standing beside the box. Old Cueball picks up a sheet of paper and checks off "Happiness", the third point checked off on a list entitled "Scavenger Hunt," where the other items include these two above that are checked off: Indian-Head Penny and Snake Skin, as well as the two items below that, are not yet checked off: Four-Leaf Clover and Shark Tooth. At least one more unchecked point is on the list, but it is covered by a speech bubble.
So finding happiness was just one item in what is presumably the longest-running scavenger hunt of all-time, considering Cueball grew significantly old during the hunt. The comic ends with Cueball asking, "What's next?"
The list indicates this is a hunt for somewhat rare items. The US Indian Head cent (penny) was produced from 1859 to 1909, making it somewhat rare. But this they have managed. (It may have been easier to find when the scavenger hunt started, depending on when that was; for instance, if the last panel takes place in the 2000s, and 70 years have gone by, then the treasure hunt started in the 1930s, when these pennies were still fairly common.) Also, the snake skin has been managed which may be a little easier to find if you live in areas with snakes as they shed their skin by molting. So finding such a skin would be the objective to find here.
A four leaf clover is a rare variation of the common three-leaf clover. A shark tooth is not easy to obtain unless you live near a beach with souvenir shops.
The title, Together, of the comic is probably a reference to the saying that you find happiness together with your loved one. Cueball could have taken many different paths to find happiness, together or separate, but he chose togetherness as a way to find happiness, which is a common theme in love stories. But happiness is not something a person finds, it is an experience, hence the need to accumulate enough experiences to determine beyond doubt that happiness was truly found. This is why he had to wait until old age before he could go back to his friends.
Note that Megan is never seen together with Cueball's friends, and especially since she is not invited to go back to them to check the happiness point off, there there is no reason to assume that she was in on the game from the beginning. She was just a means to an end, which makes Cueball's actions rather cruel and questionable.
The title text indicates that after all this time, the players may abandon the game due to being bored with it. It is typical for children to tire of a game before it is finished; except here, Cueball spent nearly a lifetime on just one part of this game! Building a treehouse is another example of a common childhood activity. Naturally, the intended mental image of a bunch of old men and women building a treehouse and playing in it like six-year-olds is another punchline. However, as mentioned in 212: Brain, the fantasy of constructing the perfect treehouse seems to nevertheless hold a permanent place in Randall's heart (or brain) regardless of how old he gets or how immature the ambition may seem. The idea of adults having a fort in the woods was also mentioned, rather darkly, in the title text of 219: Blanket Fort.
Three of the old people look very similar to three of those standing in line in 586: Mission to Culture. And much later in 1910: Sky Spotters the two birdwatchers look very much like the old version of Cueball and Megan. Giving an old person a sailor cap was also used in 2213: How Old.