After reading through Magic Jar, I've had a troubling (or exciting) revelation. As far as I am aware, there is no spell or magical item that can end the caster's possession of the original body, so long as they aren't a specific creature type. I'm aware that "creatures warded by a protection from evil and good or magic circle spell can't be possessed,"  but once they are possessed, the possessor cannot be removed as far as I'm aware.

I've looked at every spell (that I could find) that talks about removing possession: protection from evil and good, dispel evil and good, magic circle, and hallow. For the last three, they could only end possession from celestials, elementals, fey, fiends, and undead. The first spell adds aberrations to that list, but if you are not one of those, you can possess somebody and none of these spells will work on you.

I can see only three ways to end the possession:

1. Destroy the container that was used in the casting of the spell, but if that container is well hidden, nigh indestructible, or both, then this option becomes difficult.
2. Kill the host body. If your dearly beloved or trusted companion is the host, this option may again become difficult. If you have access to revivify, however, this may be more plausible, so long as you can get the 300 gp worth of diamonds and have the spell prepared in time to stop their escape.
3. Cast dispel magic on... something. I'm not sure what would be need to be dispelled, though. Is it the container, or the host body that is being possessed? If it is the host, and you are not near the container, then you will effectively kill the host, which brings you back to option two.

Is there something out there that I'm missing? How does dispel magic work in with this spell? Are there any other options? Asking in advance for one of my unsuspecting players.

Dispel magic on the possessed creature or jar would work.

Antimagic field should definitely have an effect, but I'm not sure what exactly. Person becomes normal in field? Seems unlikely without a soul. Force your soul out? That is basically dispel, with antimagic specifically isn't. I'd go with basically losing consciousness.

The wording of Magic Jar is a little concerning when killing the host body (#2). 

If the host body dies while you're in it, the creature dies, and you must make a Charisma saving throw against your own spellcasting DC. On a success, you return to the container if it is within 100 feet of you. Otherwise, you die.

Now, granted, Magic Jar uses some unique terminology that isn't well developed in other spells, and it can be a little confusing juggling the difference between "your body", the "host body," "you", and "the creature".... but no matter how I read it, the above snippet really seems to be saying that killing the host body also kills the creature itself entirely, including its soul which was in your jar. So yes, you'd have to have Revivify on hand pretty immediately after killing the host body, or resort to higher level magics later on, it isn't safe to kill the host body and then bring it to the jar trusting you'll find your friend's soul still inside. Killing the host body essentially ends the spell and kills all the various players and bodies, unless the caster's soul is within 100 feet of its jar at that moment (not its own body). 

Arguably, the spell is active on "your body" just as much as it is on the jar and the host body, and thus could be a target for Dispel Magic... but I agree that it feels like a poor nexus to throw the dispel at, and that the appropriate target should be the host body or the jar. 

If the container is destroyed or the spell ends, your soul immediately returns to your body. If your body is more than 100 feet away from you or if your body is dead when you attempt to return to it, you die. If another creature's soul is in the container when it is destroyed, the creature's soul returns to its body if the body is alive and within 100 feet. Otherwise, that creature dies.

I do read that clause of the spell to treat "container is destroyed" as equal to "spell ends," and not to say that you can only save your friend by destroying the jar, and not by ending the spell.  That may not be the strictest reading of the spell, but seems like the intent of that paragraph. 

While I agree with everyone's interpretation of dispel magic working on the host body, it seems a little unfortunate that a 3rd level spell works where the only other spells that get close are 5th level. I guess the incentive of not killing your possessed best friend in the process is a pretty good deterrent, though.