They're young enough that you can make just about anything into a fun vacation with the right amount of planning and excitement, especially for the two youngest.
Pick something that "breaks the rules" for vacation. example: if you don't have dessert every day or if your kids like something that you don't let them have every day you go on an "ice cream tour", "slushy tour" or even "mac and cheese" tour and look for places you can go and try something new and everyone votes on their favorites.
Are there areas near you that have nice local parks? One day I got a request to "stop at that park" and it was someone's backyard swing set. If your kids like playing on playgrounds you could go to a nice area that has playgrounds and visit one or two each day. Good news with this one is they will probably sleep really well from playing outside all day.
Many of the amusement parks in Ohio have a Snoopy area that have many kiddie rides. I'm not sure where you are in Michigan, but
Cedar Point in Sandusky might be not too far away. Everyone's "budget" is different so that may be more than you want to spend.
What do your kids like to do? Are there free or donation museums you could travel to?