Activities for 5 Year Boy Birthday Party at Home and in Winter

My daughter loves snowmen so we used to have a Snowman Party.
Pin the nose on the snowman - we blindfolded with bandanas that they got to keep.
The magic snowball game - wrap a tiny toy in a ball of white streamer - they all have to unwrap it -- they get to keep what they find.
The snowball on the spoon race - it's a marshmallow on a spoon. Teams compete to walk with the snowball down the hall and around the chair and back. They transfer the marshmallow without using hands.
Decorate your own cupcakes instead of a birthday cake - they love it, but it's messy.
Snowball soccer - clear off your dining room table. Get a pingpong ball. They have to blow the pingpong ball off the table to get a point. It's hysterically funny!!
Enjoy!
LBC

I always tell the parents to bring the kids' snow gear I have a fenced in backyard so we let them go out and play in the snow for awhile. My 8yr old bday is before christams so after they come in they get some hot chocolate and decorate something. One year I little stockings that were also used as the goody bag, shatterproof ornaments for them to decorate, gingerbread cookies, little gingerbread houses. then we do cake and presents and the kids play inside for a little bit before wrapping the party up.

I agree with Rachel.

You could also do the following:
Make marshmellow igloos.
Make paper snowflakes
Create and 'avalanche' by putting some Fluff a bowl with buried treasure and the boys have to rescue it.

My son is a December baby and we have had good luck doing pinatas inside.

Updated

also: pin the tail on the donkey and musical chairs may be fun. booking a party at another venue may be a good alternative as well (like at a restaurant, an indoor pool etc).

Kids love BINGO. I make bingo cards at home on my computer; it is a little work since each card needs to be different. I use pictures instead of numbers to coordinate with the theme of the party. Last year the theme was Mario, so I used pictures of all the characters/items in the game. This year the theme is sharks, so I used different sharks, characters, and body parts (fin and tooth). Of course there are small prizes for the winners that I picked up at the dollar store or craft store. (I usually play until everyone wins). Good Luck.
G.

You could have the kids design their own t-shirt. For my daughter's 5th (who is also a winter baby) all the children received plain white t-shirts that said "I had a blast at ______'s 5th birthday party" and they were able to decorate them however they wanted with fabric paint, markers, etc.

You can have a karaoke contest and videotape the performance for posterity (LOL).

You can play games such Twister and the like for small prizes, or even musical chairs. I wouldn't suggest any game that takes more than 15-20 minutes to complete.

There are any number of things that you can do inside that will be fun for the kids. And as long as your son is having fun and creating memories with his friends, the rest is secondary. All he'll remember years from now is the fun he had.

My son has a Feb birthday so I know what you are gong through. I always put out a snack table (baby carrots, grapes, gold fish, cut apples etc) so they can feed themselves and had theme parties with goody bags going along (Star Wars, Lego's). Some parents are great with the whole 1. we do this 2. we do that but I have always just let the kids play and this past Feb when I had the Lego party I gave them each a Lego person that looked like them, make a Lego cake (Bettycrocker.com has instructions) and drew an outline of a Lego person on 8x10 paper for each kid to color in (they really loved that!) we had pizza and cake and every one have a good time.
If he likes Cars do that theme, it can be inexpensive and not mind blowing to just do little things for a fun party..a Pirate party is fun too

We have lots of Winter birthdays too. We usually go to the local kids gym and rent it out and let the kids have their party there. Our kids take gymnastics and dance and they love inviting their friends from school to their party at the gym.

The owner or teacher does an hour of gymnastics with the group then they go back to the break room area for the cake and other stuff. The gym provides the tables and chairs and sometimes the decorations. I don't like balloons so we don't do them. It costs about $100 and there is little to no stress for me. I don't have to manage kids, entertain them, clean up after them, etc...

We also have a place for little "Princesses" to go have an afternoon tea. It's very Fru Fru and they get to dress up in fancy dresses and shoes with tiaras and put on makeup and everything, then they get up on the tiny stage and sing along to their favorite Disney songs or some other age appropriate music. It's an extreme amount of fun. The girls love it and it's always a treat to see them get up and do Karaoke.