Runescape as we know it contains a large amount of content, with 25 skills and a max total level of 2,496. That comes to a whopping 417,099,511 xp to have a maxed account (120 in Dungeoneering and 99 in the rest). But you still have minigames, and a little thing called Distractions & Diversions.
Distractions and Diversions were officially introduced into the game September 2nd 2008, with a current total of 15, all of which are members only. The current 15 are the following:

• Ancient effigies
• Balthazar Beauregard’s Big Top Bonanza
• Champions’ Challenge
• Court Cases (Most Recent)
• Evil Tree
• Familiarization
• Fish Flingers
• Managing Miscellania (& Etceteria)
• Penguin Hide’n’Seek
• Phoenix Lair
• Shattered Heart
• Shooting Star
• Tears of Guthix
• Thieving Guild
• Treasure Trails  (Oldest)

Each one provides unique rewards for completing them, and a limit to the number of times you can participate in a period (typically a week).
There are a few questions about the purpose or necessity of these Distractions and Diversions, especially the newer ones, and are they really worth doing?

Disastrous Diversions
These are the worst of the bunch, the runts and the misfits. The activities that were put in with good intentions, but were badly applied, are just plain useless or aren’t fun.

Champions’ Challenge
This relic from 2005 has almost no redeeming values to it, other than the fact that they can be a pleasant surprise monster drop. The only useful reward for defeating a champion is the xp gained both during and after, as the banner-waving imps are not only pure aesthetics, but somewhat annoying in an already overloaded GE.

Familiarization
Ahhhh, the long awaited and completely underwhelming Summoning D&D. I highly recommend avoiding this, the rewards are desultory and waiting for, finding and getting to said obelisks can be a pain or (depending on your levels or quests completed) impossible to get to at that moment. The rewards consist of a choice between triple charms for a short time (with time being deducted for each charm picked up) or a “Box of summoning ingredients” that contains random item used to make pouches depending on your summoning level.

Fish Flingers
It is slightly enjoyable, but offers no bonuses other an absolute max of a little less than 20k xp and tokens to eventually buy tackle boxes that have no other function than saving a few bank spaces.

Asa’s Recommended Diversions:
If you are interested in doing any Distractions and Diversions, these are the ones I both like the most and seem to have the best rewards:

Ancient Effigies
These relics both provide immediate xp when “fed” knowledge, but also provide a dragonkin lamp when completed, which can either be sold or used for a decent amount more of xp. If you don’t have the levels to complete, you can use the assist system.

Managing Miscellania
This is one of the older activities rebranded as a D & D. This is my personal favorite, as it requires little effort and can have a monetary return of approximately 540% depending on how much money you keep in the coffers, your approval rating and the distribution of your workers. It’s recommended to check it once a week to maintain your approval rating and to collect your resources. To reap the full rewards, do both the “Throne of Miscellania” and “Royal Trouble” quests.

Penguin hide’n’seek
This D & D doesn’t require the completion of the “Cold War” quest, though it is highly recommended that you complete both it and “Hunt for Red Raktuber” to be able to gain more points. The main draw of this activity is the fact is you can quickly spy on a penguin if you happen to stumble across one. Each penguin spied on gains you 1 penguin point, though some penguins are harder than others to locate due to the existence of quest related locations. The xp reward can be quite impressive if a decent amount of penguin points have been accumulated. The best part of it is the spontaneity available and the generous xp reward.

Tears of Guthix
Tears is another one of the relics, and the oldest of what are now D & D’s. It can be repeated after 1 week with a requirement of either 100k xp or 1 quest point gained since the last visit. You must do the quest of the same name before access is granted to the tears. The main reward is the xp granted to your lowest skill, depending on both the level of that skill and the number of tears collected. The more quest points you have, the longer you will have to collect tears.

Thieving Guild
The Thieving Guild (where you can get hanky points) in Lumbridge is a convenient way to gain easy thieving xp once a week by exchanging hanky points. To gain the full benefits of the guild, it’s recommended that all capers in the “Buyers and Cellars” quest.

Treasure Trails
If you are a member and have been playing for a while, chances are that you’ve at some point received a Treasure Trail clue. Treasure Trails are the oldest of all these activities, debuting in 2004, and has the most unique rewards. The max amount for a single clue is currently estimated at 100,000,000 gp. They can be dangerous, and annoying, but can also yield fantastic rewards (for example, the crazily over-priced 3rd Age armor sets).

Everything in-between

Balthazar Beauregard’s Big Top Bonanza
Mostly unremarkable, other than the fact that some of the costumes look really sharp (especially the ring-master costume) and it’s quite unique.

Court Cases
The newest D & D out there. It require some intellectual work with interesting results. It has mediocre rewards, but interesting dialogue in the court-room, plus it’s fun to play the detective and gather evidence for the case.

Evil Tree
The Evil tree is also interesting in that is can also yield some very unique Woodcutting rewards along with both wood and birds nests.

Phoenix Lair
This requires the completion of the quest “In Pyre Need” and is not for the fool-hardy, you can die in the process of resurrecting the Phoenix. Main points of interest are the possibility of getting either a cute or mean phoenix eggling and phoenix quills to make the summoning pouch.

Shattered Heart
Little rocks appear when training certain skills to create a statue that, once completed, can be put in your PoH. Nice for being un-involved and easy to do with just normal grinding.

Shooting Star
All it requires is a pick, a little luck, some speed and a telescope in a PoH. Nothing too noteworthy in my opinion, other than the fact that it involves aliens.

As you can probably see, there’s a great variety of D&D’s out there, and these are just my suggestions for what you should do, but like with skills, you should stick with what you enjoy the most and feels the most worth it.