Timing is everything. It's always a challenge leaving a fun place with toddlers, but you can't let that keep you from taking them places (although I'd like to just stay home using that as an excuse). One of the ways to successfully leave is to follow your toddlers' lead. This requires a lot of time and flexibility, but sometimes it can be pulled off flawlessly.
First, you have to wait until your children just barely start to get bored. When the pauses between activities become just a little longer than they were, start subtley packing up. If your kids are still having as much fun as they were in the beginning, it's going to be exceptionally hard to drag them away without a tantrum. On the flip side, if you let them get too bored, they'll quickly move from ready-to-leave to hellion-mode, allowing the boredom to get the better of them and you.
Yesterday, I took the babies swimming. We splashed in the kiddie pool. We played on the steps of the big pool. We filled buckets with water. We danced around the play area. Someone came out of the gym, and the babies set to offering him leaves and buds from nearby plants. I noticed, though, that they started listlessly looking around, waiting for their next fun opportunity to arise. As those opportunities dwindled, I started packing up. I know from experience that uttering the words, "let's go home," creates instant turmoil. Even if they were ready to leave, once those words are spoken, they will insist they were in no way ready yet. So, instead, I told them to put on their shoes. I pointed out something fun just beyond the pool gate, and we left the pool quietly.
Had I tried to leave before they were done giving the stranger leaves, or splashing in the pool, I would have had floppy toddler fish on my hands. If I had waited any longer, the games the babies were playing to distract themselves would have taken a more daring, more dangerous turn - something a mom with twins by herself in a pool area cannot afford. When one twin runs one way and the other another way, it's usually cute to onlookers and only slightly annoying to me. In an area where there are large bodies of deep water on either side, two kids running in opposite directions is harrowing at best, disasterous at worst.
So, when you need to leave a fun place, start paying attention to your kids' cues. They'll never tell you their ready to go, but they'll slow in their innocent endeavors as they start to think of more devious ways to spend their time and amuse themselves. And if you think this is a large window, you'll be unpleasantly surprised.
I'd say you have anywhere from three to nine minutes to actually leave a place once you see a fit of slight boredom set in.
Ready. Set. Go!