First of all, take a look at this image.
http://www.byond.com/members/TheMagicMan/files/dung.png
It is currently the dungeons in a game I am working on. Of those the first two are completed, the other 3 are half done.

Now, I will explain these dungeons.
The first one is tiny, has very little to do but is a beginners dungeon. You might as max spend 5-10 minutes in it.
It took 4 or so hours to make, this includes layout, monsters and quests.

The second one is bigger, is a relatively simple maze, but will still take no longer than say... 10-20 minutes to finish.
Took me maybe 6 hours to make.

The third dungeon is a much more complex maze. It is also the first dungeon where enemies start getting smart and doing more than just attacking.
At the moment the layout is more or less finished, most of the monsters implamented, but no quests at all.
I estimate it should take no longer than 1 hour to finish, when all quests are implamented.
Has taken me 4-5 hours to make so far, adding the rest of the monsters/quests will bring that close to 10 hours.

The fourth dungeon is nowhere near finished. It's layout is only 50% done, it has some monsters, but no solid bosses (will be 5). No quests either.
Has only taken maybe 2-3 hours so far, but again, finishing it will bring this upto at least 10-12 hours.
Estimated time to do everything in the dungeon? Maybe 1 hour or so.

The final dungeon is mostly finished. It's layout is done, and so are all the monsters (11 progressively difficult bosses).
Due to the way it is designed to fight the first 4 bosses you'll need to be level 20, the next 25, then 35 for the next 3, then 45 for the next 2, then 60. However, if you beat the 4 bosses at level 20 and come back at level 35 you do not need to beat them again to progress in the dungeon (they drop keys to the next part of the dungeon).
It at the moment has no quests however.
I estimate it will take no longer than... 2 hours to finish this entire dungeon.
I've spent 2 full days (maybe 15 or so hours) working on it so far, mainly due to some pretty excellent boss AIs. Adding quests will knock that upto maybe 18 hours.

So lets take a look, if I finish all these dungeons I'll have spent about 50 hours working on them. But that translates into roughly 4-5 hours worth of playtime for the average player.
Which works out to about 8-10% of the total time I spend working on this game translates into playable time with things to do.

So here is the point of this topic. What sort of things should I add to this game that will take less time to develop, but translate into much more hours of playtime WITHOUT this added stuff turning into a grind?

Secondly, just how much of a grind can you put up with? At the moment I am trying to minimize it.
At the moment, excluding quests if you kill equal leveled monsters you will need to kill 8179 to level up to the maximum level.
If you take quests and boss monsters then that number is cut in half. This translates into 40 kills per level up on average (in actuality it is 9 at level 1, and something like 140 at level 99).
Is this too much of too little of a grind for you?