My eldest was born in late October, so we have done a couple things over the years. The first year, we had a cowboy themed party and the kids dressed up in cowboy costumes (a nod to Halloween, in that there were costumes, but we hadn't yet decided how or if our family was going to celebrate Halloween yet). We laid out haybales, all the kids got little vests and sheriff or deputy stars, little plastic pistols, and there were a lot of cowboy themed games for the older kids. Joseph, having just turned 1, was more involved in the pull string pinata, being passed around to friends and family (many, like my grandparents, uncle, sister, and older friends were meeting him for the first time). He had his own smash cake (came free with the sheet cake we bought from Walmart's bakery); the sheet cake was a pretty little western scene with horses and fences, a couple trees (which 5 years later, they STILL play with).
We did decide to go to a Halloween party 10 days later, so we did decide to do some mild Halloween stuff. At that party, it was all for little kids. There were big spiders with smiley faces, fun music like "Monster Mash" that we all danced to, lots of autumn decor AND candy apples, caramel apples, popcorn balls, candy corn stuff. Some little ones were dressed as pumpkins, or Dorothy, stuff like that. My son was JUST learning to walk and held his arms out straight in front of him and would lean forward, then his legs would be all stiff and he'd basically just stumble forward 5-7 steps at a time. We joked that he walked like a zombie. So I took some green and brown halloween makeup and basically just smudged his face up a little, put some gel in his hair and messed it all up so it was sticking out everywhere, smeared some make up "dirt" on one of my husband's old white undershirts and tore it up a little, and that with his little jeans was his "costume". He was really cute though. They played musical chairs, hot potato, simple little things. THEN we all went as a big group trick or treating (dont forget the wagon because they get tired of walking fast and parents may not want to carry all the way around the block).
We've had other seasonal parties where we went to a pumpkin patch and played in the bounce house (I went in with him because he was only 2 and I didn't want him getting hurt), we went on a hayride, watched the "big green tractors" do some work, did a pony ride, a corn field maze, picked out a pumpkin and took it home. I roasted the seeds, made a pumpkin soup, and we made a jack o lantern (with these cute little battery operated "flame" lights that flicker, but aren't real candles, so safer). We went to Michaels and got a little fake pumpkin and dated the bottom, and used a little decorating kit (black cat with purple jewels, pipecleaner whiskers, etc).....we started that on his 2nd Halloween and make one every year, date it, and put them all out. Instead of a birthday cake (because he didn't really care---didn't eat cake enough to know the difference), we made a delicious pumpkin roll (the ones that have cream cheese in them) and put 2 candles in that instead.
Another birthday: we planned a party to be the same day as a church's HUGE fall fest. We had pizza, some silly "gross" Halloween food/drink, "built" scarecrows with clothes the kids had outgrown, ate a big pumpkin decorated birthday cake and sang Happy Birthday to him, then the kids got ready together in their little costumes (my son was easy: a dinosaur costume), we went to the big fall fest for games, pony rides, a little miniature choo choo train, bounce houses and slides, etc. We thought we were really getting away with something not having to pay and run all that stuff, but got to have much more than we would have had if we'd had a party at home, just attending the fest instead.
This past year was a Star Wars theme party where the kids got to dress up, play all kinds of games, had all kinds of themed drinks/foods to go with it all. But then while the kids were eating their pizza suddenly we heard the imperial march and Darth Vadar surprised us! The kids had to chase him around the backyard, beat him up with their foam sabers (that I made of pool noodles and papertowel tubes) and they turned him back to the good side. I'd rented a very good Darth Vadar costume (for husband to sneak off and put on) from a local costume shop and got a little discount for returning it a few days before Halloween.
We liked to mix "seasonal" with a little Halloween mixed in. I'm sure that Pinterest, as someone else mentioned would have all kinds of neat ideas. Also, look up "kids halloween birthday party" and I'm sure you'll find tons.