Cars, trains, trucks on the floors, on the shelves, on the walls, on the bed, everywhere an adventure might happen. Art work, puzzles, take them to the park for a couple of hours at least, play with Play Doh.
You can do pretend play too. They might like dressing up like doctors, firemen, make forts with blankets and kitchen table chairs then play explorers on the way up a mountain, build with blocks whether it's cut pieces of wood or Lego's (they should enjoy using their mechanical and spatial skills), they'll think you're the coolest adult ever if you mention snot, boogers, you know, normal stuff Mom's won't let them talk about.
http://www.sheknows.com/parenting/articles/844893/lessons...
Make Play-Doh or some other experiment kind of stuff like Cornstarch and water with a few drops of food coloring in it. They can stir it up and watch it change.
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Messy play, parents usually hate the idea of doing this with their own kids because it takes so long to clean up the mess, you can do stuff like these ideas then go hose the kids off and the play space too if you do it outside.....
Fingerpaint with pudding on the kitchen table (if it's smooth and sealed so it's slick), or use shaving cream with some jello sprinkled in it to make colors, watch the 2 year old so they don't put anything in their mouth to taste it. Having the little one write letters, make fish, make squiggles with the side of their hand, making clouds in the sky...anything interesting and named will make this fun for the little one. The 4 yr. old will enjoy writing his name and making fart sounds with the palm of their hand laying flat on the moist table and pushing it up and down...boys must play about pooh and pee and silly stuff.
http://baby.more4kids.info/2008/03/benefits-of-messy-play/
http://www.ehow.com/info_###-###-####_tactile-ways-write-...
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Dig in the dirt and look for worms and bugs. Go for a walk and collect rocks and sticks and whatever they like. If you simply hold the 2 yr. old's hand instead of a continuous stroller thing they'll get more out of the hands on nature stuff but taking the stroller would be good so if they get tired you can plop them in it.
Sensory play is important and they'd love it.
You can plan ahead by getting a flat style storage tub and putting some sand in it. Then put stuff in it that is buried. I would bring the sand separately so it wouldn't be so heavy to carry. You can take the tub and put the things in it, again, 2 year olds will put anything in their mouth if they take a notion so make sure the items are big enough to not slide down a toilet paper tube, then put a layer of sand in, then a few more toys, then more sand. The deeper the better but not more than 10 inches or so, the little ones may lose interest if it's too deep and too hard.
You could do pasta, rice, popcorn, egg noodles, measuring cups, bowls, spoons of different sizes,
http://childcareandbeyond.tripod.com/sensoryplay.html
http://pinterest.com/rarollston/ot-tactile-sensory/
http://edutask.com/child-development-education/sensory-pl...
http://notjustcute.com/2010/03/24/a-handful-of-fun-why-se...
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If it's warm enough you can set up the sprinkler and let them run amok in the water. Or take them to the local splash pad, I would NOT take them swimming since you don't know them extremely well in the water already but water play can be very fun. Anything to keep their minds occupied and full of fun to let mom rest and have some alone time with the new baby.
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What kids learn while they are playing, a very good article about playtime.
http://users.stargate.net/~cokids/Borden.html
Some good ideas for kids that you're not too familiar with, they may think some games are fun but too aggressive.
http://www.healthychildren.org/English/ages-stages/toddle...
I think you're a very good friend to take on 3 boys for your friend. I think I would plan on the park for at least an hour with a big bag of sand toys and a cooler with snacks in them. Then if it rains plan on a lot of indoor activities.