There are no rules that say that in order for it to be a date it has to be with someone else. Don't think that because you are single you can't take yourself on a date instead. These 6 date ideas work for both solo dates as well as taking your significant other out.
Apple Picking/Pumpkin Patch/Farmer's Market
A quick Google search will show you where the closest apple orchard or pumpkin patch is if that is something that you really want to do. Some farmer's markets aren't open all year but the ones that are open all year are really a good way to spend your day. You can usually get the apples and sometimes pumpkins here without the hassle of picking them yourself, especially if this is a solo date.
Go for a Hike/Drive
Get in the last hikes of the season while the weather is still warm enough. This doesn't mean that you have to plan an elaborate hike, but rather go for a nice walk, and spend 2 hours hiking not all day. Find the hike that fits your pace and simply enjoy nature. The same can be said about going for a drive, get yourself a warm drink and simply take a drive. This can be backroad with no destination or plan an activity for the end destination. You don't have to plan for all day, but at the same time, you plan for what you feel like doing. Want to make it a weekend trip, do it; only want to be gone a few hours, do what works for you. This (in my opinion) is the perfect time for road trips. If you are doing this solo make sure someone at least knows your plans. It's always good to have someone know where you are, and where you are headed.
Baking or Cooking Your Autumn Favorites
Get your favorite recipes together and spend the day baking/cooking. This can be combined with the next idea as well. Pick 2 or 3 recipes and make them. Plan a meal, or just make desserts. I think that this is the perfect way to spend a rainy autumn day.
Movie Marathon/TV Show Binge
Pick your favorite movies or TV shows, whether that's Halloween movies, autumn movies, Disney, or your favorite TV show, etc. Spend the day with them either playing in the background while you bake or cuddling up on the couch and simply relaxing. If you like me I typically start watching Christmas movies at this time of year (I know that Halloween hasn't even passed yet, but I love Christmas). You don't have to spend all day either, plan an afternoon activity and spend the morning watching movies or vice versa. Again I think that this is the perfect rainy-day activity.
Library or Bookstore
This is an activity that would be (in my opinion) better as a solo date, unless both you and your partner are readers there is going to be someone not having as much fun. I love going to the bookstore and to a lesser extent our public library. Get yourself a hot drink (or cold) and simply browse. If you find a book you really want, treat yourself and find somewhere to sit and enjoy your drink and read your new book. I find bookstores extremely relaxing and a good place to reset if you are feeling stressed. (at least for me)
This is really only a good idea if you enjoy cider/wine. But these can be a fun activity if you have someone to go with for sure. And who knows maybe you will find your new favorite drink. This would work as a solo date. Who knows maybe you'll make new friends.