I'm trying to keep my kids involved with the pregnancy as much as possible although they're 7 and 9 so they understand more than a toddler would. maybe have your kids read bed time stories to the baby... even if they're too young to read they can make up a story to go with pictures :) ask them to help give baby a bath or hold the bottle while feeding if you use a bottle etc. Just try to have them "help" as much as they want, and "if" they want. Also baby will sleep a lot the first few weeks especially and do little things like art projects or story time with just them while baby us napping.

I saw someone build a build a bear, and give it to the older one "from the baby" at their birth :) something from baby to older brother or sister is super cute

I let them participate in any way possible from feelings to diaper changing, even if they're just passing me the wipes and diapers. I agree with @marienoelle about the presents. I plan on getting all my girls new stuffed teddy bears and saying that it's from the baby!

thanks @marienoelle I'll definitely try it!!!!

I'm not a momma of any older kids but I've read cute things about keeping them as involved with the baby prep as possible, one article even suggested getting them a big sibling present to give to them at the hospital after baby is born :) lots of mommas I know also say they try to designate one special thing or time of day no matter how tired to do with the oldest child (ren), like a bedtime regimen or dinner or something to make them feel involved and still independently important. good luck!