Looking for Activities for a Toddler

Or son always loved making forts - and he could make them out of just about anything.
An easy fort is one you make out of a large cardboard box - get one from an appliance store.
You can cut out windows and decorate it anyway you want - color it, use blankets/pillows, make it a rocket or submarine or castle (or anything!) and then read stories in it or have lunch/snack there or even nap in it.
When it wears out - throw it into recycling!

We always tried to do lots of outdoor activities like tobogganing, ice skating and snowshoeing. We visited the zoo and museums, drove around and looked at Xmas lights, went to the library, went to a movie and a live theatre performance or concert. We also went bowling, indoor mini golfing and swimming at the indoor pool. There was always more to do over the holidays than we had time to do! I live in one of the coldest cities on the planet, so I doubt it is too cold to be outdoors in PA.

ETA: A four year old is not a toddler. Toddlers are 1-2 years of age.
ETA2: Sorry to point out that your child is not a toddler, but if you ask for/receive suggestions for toddler activities, a four year old will probably not enjoy them very much. Four year olds need a little more stimulation, excitement, adventure and activity than a toddler!

I suggest story time at the library, cooking, baking and crafts (painting, play doh, etc.) at home, playing outside in the snow (sledding especially, that was always my favorite!) and hanging out with other moms and kids in a playgroup or just with your own friends.
And honestly why not just hang out and not always be "doing" something? Some of my best holiday memories with my kids back when they were little were just cuddling on the couch binge watching Disney and Pixar movies.

Here are the things my littler one is into this week:
Play doh
Little people
A version of soccer - me and the 2 kids in the back yard (yes even today when it's only 40 degrees)
Science Center/Natural History Museum
Candyland
Chutes and Ladders
Go Fish

Usually there's a website or Facebook group you can join for kids' activities going on in your area. Here there are a bunch.

I don't drive these days but even before that, I didn't do a lot of excursions with my kids - they played mostly. If you want to do a craft with them, there are so many places you can get ideas and just hit the dollar store for supplies. Mine would paint or glue.

I'd make up a batch of homemade playdough and give them a rolling pin, muffin pans, etc. let her make a bakery. Pretend restaurant, etc. Mine were big on imaginative play.

Dress up - again, dollar store or just your own closet, old Halloween costumes, etc.

Getting together for coffee and kids with friends ..

Forts (we always had a fort on the go). Let her watch a movie in it and give her popcorn, flashlight, etc.

I used to pull out one big toy a day or two for my kids. Something they could play for a few hours. So one day would be Little People toys/village, etc. Put it away and bring out something fresh the next day.

Teddy Bear picnic. Just throw an old blanket down and have lunch on it.

Mine were just happy to come out with us to do errands. We would hit a donut shop on way home as treat - hot chocolate.

Lots of ideas for excursions below if you want to get out of the house. My kids loved those too.

At age 4 my kids loved going outside. Take her to a nature area for a "hike". Do an art project together. My daughter loved to create art with me where we both would work on it together. I got a big roll of paper and we would sit on the floor and finger paint together. Make prints of leaves or veggies or whatever you can think of. Take her to an indoor pool. This is always a favorite with my kids! Take her ice skating. Both my kids started learning at age 4. Go to a museum...they all seem to have kid areas nowadays. Go see Moana. Ride the bus somewhere. I remember at age 3 and 4 my kids thought it was super fun to ride the city bus. Collect sticks/leaves/things in nature and make a nest. My daughter was really into making nests and fairy houses at age 4. Make homemade play doh. Drive somewhere new you have always wanted to go...yesterday we took the kids to a historic fort about 30m away we had never been to. They had a blast running around and checking it out. Take her sledding if you have any snow. Take her to a children's theater production. Take her to a paint your own pottery place and let her paint her own bowl or plate.

Public library - story hour or just browse the books on display. Usually there is a theme (probably winter and Martin Luther King right now). Get a book (even an old 1950s or 1960s style, before the internet and video games occupied kids) - do science "experiments" or put an avocado seed or potato piece in something to watch it germinate.

Is there a children's, art or science museum nearby? We always had a membership so we could go for free - and it eased the sting if our child got tired of it after an hour or so. We saved a fortune even if we only went once a month.

Check the craft store for little workshops. Even if there isn't a program, talk to an employee. Buy wooden shapes and paint them (my stepdaughter displays them on top of door and window molding) or put foam shapes on them, or make picture frames to give as gifts. Paint wooden birdhouses and put them outside, with dryer lint inside as nesting material.

Bake. Take the extras to the fire department or police department. In our town, you can visit the fire department with a child anytime you want. Unless there's an actual emergency, kids can climb in the engines and ambulances.

How about including her in the things you need to do daily? Get her a little broom or a Swiffer duster, and while you vacuum or mop, she can dust safe things (books, non-breakable things, baseboards) or sweep something.

She can help you match socks. Make it a fun game.

When you cook meals, get her involved. Teach her about measuring. She can tear lettuce for a salad, help measure flour, stir batters, etc. She can help you grocery shop and learn to read labels. ("Find the butter label that says Challenge on it, or Land O Lakes").

Or, if she is especially interested in something you do (sew, cook, bake, etc), get her a playset of her own that she can play with alongside you.

You don't need to occupy her time all the time - just chat with her and perhaps plan one vacation outing, to a movie or the zoo or a children's museum and have that to look forward to.