Team Fortress 2
Team-based games are the perfect fit for MMO gamers, and you really can't find many better than Team Fortress 2. In addition to that, the game is full of achievements that'll keep even the most completion focused type busy for a long time to come. If you don't already have the game yet, it's only 20 bucks, unless Valve is selling it at half off occasionally when they do a substantial free game update.
The free updates are actually another thing Team Fortress 2 has in common with MMOs. The most recent being the Sniper/Spy update that also added a twist on a game mode added a year or two back. Even if you've never been the biggest shooter fan, you'll probably come out liking this game a whole lot. You don't even have to shoot people, if you don't want, as the Medic class comes with a medigun that heals your teammates and eventually charges up for temporary invulnerability -- or 100% critical hits if you've unlocked the second gun.
There's also the Engineer, who builds defenses and utility devices. Between those two classes, you'll find plenty of non-shooting to do, although everyone eventually succumbs to the fun of the other classes.
And yes, I did say unlocks. You can unlock tons of secondary weapons, and they just recently added cosmetic hats that are unlocked as well! Really, even if Jumpgate Evolution and Champions Online were still coming out in summer, you should be playing TF2.
There may be no teamwork here, Fallout 3 does offer a massive open world to explore. Every MMO tries to cater in some way to the explorer players, but this game is their paradise. There's something about a largely abandoned toy factory that just screams, "Come and explore me!" Plus, randomly wandering the capital wasteland will earn you all sorts of strange and wonderful experiences. No spoilers here, though.
The combat -- while primarily of the turn-based flavor -- doesn't always have to be paused here and there, you can play it shooteresque if you want. Although, my personal advice would be to play it like an RPG over anything else. And if you really don't like the idea of RPG and guns, then just go melee and whack stuff endlessly!
There have already been three 9.99 content packs released, the most recent one being Broken Steel, which opens up the endgame, adds new side quests, ten additional levels and plenty of new skills to unlock for your character. They aren't done with just three, however, and two more are being released soon -- one in June and another in July. Each update contains new armor, weapons, skills and quests to enjoy. So basically, you'll have new stuff to do all summer long!
Dungeons and Dragons
An old standby for many, Dungeons and Dragons is the perfect getaway for people looking for something that fits like an MMO, but with a much more personal flavor. Plus, 4th Edition is perfect for new players who're familiar with MMORPGs, as many of its more inspired mechanics will be instantly familiar in many ways. And for players who aren't fans of the newest edition, there's always older editions or tabletop alternatives like Shadowrun or White Wolf, too.
Summer is the perfect time to get back into D&D this year, too, as the new Eberron books are coming out June and July. Eberron, for those unfamiliar, is easiest described as dark fantasy punk, with magical robots, trains and the like. But if you're looking for a more high fantasy setting the Forgotten Realms books are already out, too.
These are the games I plan on playing over the course of summer. And who knows? Maybe I'll go back to an older MMO as well, but right now I'm really just waiting for the shiny new non-fantasy games. Sometimes you need a break from non-stop MMOing, so that when you come back to playing them it's all the more intoxicating. I'm not sure how long I can fill my need to level, PvP and group with friends without any kind of MMO -- but you can be sure I'll keep it up for as long as possible.