Be prepared to be bored out of your damn mind, periodically delighted, and always realizing that yes they somehow found scissors while you looked away long enough to see what time it is. Note: 6 hours haven't passed, it just feels like it.

You can go to the park and come home 6 times. Each time will be an adventure.

They love water. You need to supervise but pouring liquid between vessels is fun. Same with sand. You'll probably need to change their clothes 3-4 times anyway so it doesn't matter if what they are doing messes up their outfit. DO NOT leave a kid like this unattended if they are not confined in a very, very safe place. Ask parents what they do when they need to use the washroom and do that.

Make sure you have a good line up of parent approved snacks and drinks.

You might be able to buy some time with a video but better is to get some raffi songs going and show them how to clap, have a dance party (well away from stuff they can hit their head on etc - you can do this seated).

Figure out the potty training situation but kids this old get upset when away from parents and can regress so could mean they have accidents. If this happens don't get angry. Just change them and back to playing.

Good luck.

If you have trusted friend or relative they can watch them while you use the washroom or google how to get you keys out of the toilet (do not give them your keys or phone - memorize that).