My kids are 18 months apart. We wondered about our son right away, too. I made sure I included him with everything with the baby...when I'd nurse, our son would play beside me. I'd sit on the floor to nurse, so it wasn't so far removed from him. When I changed her diaper, he could come "help." He'd stand on a stool next to me, hand me a diaper, or whatever. (Thankfully, he didn't care too much about this part). We'd read books while I nursed, after I nursed, etc. When the baby slept, I tried to get him to sleep, too, so that I could sleep. The other thing we did for our little man was to make sure he got Daddy time in the evenings. We'd even call it "Daddy time" so he would know this was special for him. In that time, Daddy wouldn't mess with the baby. Or, if he did, it was daddy and son time with the baby. I think having our son watch Daddy be kind and gentle with the baby girl helped our son to want to be kind and gentle, too. At least that's how it turned out! They are 11 and 9 1/2 now, and are mostly best friends...when they're not worst enemies. I must confess getting out of diapers was very freeing for me!
I'm excited for you. You'll be busy at first, but it is sooo worth it to have the kids be close and friends and active in each other's lives now. Congratulations and good luck!