I would go for a local park, where there are often BBQ facilities available too

I had a Bring A Plate party for my husband. The theme was 1970s so you had to bring a plate from that era. Had some very imaginative plates with no double-ups. Most people did quite a bit of homework. It was a great success and everyone had a great laugh thinking back to how some of these dishes were extremely fashionable.

Local park. Everyone BYO drinks and a plate of food. Too easy

What about paintball? He could have a blast doing all the bro stuff with the boys! And afterwards a nice BBQ dinner or lunch in the backyard or one of his favourite places. Then you could do snags and rissoles, kebabs, etc. BYO drinks.

Theme party …how about Aussie. Can have a BBQ in the park with snags and a camp fire inspired cake (google) . Everyone dresses up and this can also be cheap eg jeans and a flannelette shirt and hat with corks around it.
All the best.

What about an afternoon in a local park or similar. BBQ’s are often available and no cost for the space. I guess all is dependant on restrictions though

My daughter in law recently had to cancel my son’s party & ended up ordering a seafood platter from a up market seafood restaurant & they both got dressed up & enjoyed their intimate dinner at home! Even their kiddies were feed early & then quietly watched a movie while they dined in style.

Guess it depends on your area’s COVID restrictions. Definitely outsource the venue – way to much work at home.
If you’re somewhere rural / warm – a barn you could hire?
Social bowls at a local club?
Scavenger hunt in a large space?
If you’re thinking a meal then either massive pots of a couple of stews, a BBQ or ask people to bring something to share. Definitely BYO alcohol though if you’re on a budget.
Or would a local vineyard / brewery do a reverse BYO (people bring their own food/ picnic but buy their drinks from the venue)?

What does your husband like doing? with COVID there could be a couple of parties. Like a progressive dinner or a group scavenger hunt.

100 people! That’s huge. With COVID restrictions changing all the time it might be best to plan something outdoors or where there is an option for some people to be outdoors. A local community hall could be a good option that has a park next to it.

Hire a room somewhere – local community Hall or Scout Hall – or have a BBQ at the beach or in a park!

Hire a hall through your local council, bulk shop at aldi or costco, use simple upcycled materials for decorations

Hire a room at a small bowling club – people cant outstay their welcome and your house wont get trashed since the party isn’t there.
See if they can cater and do finger foods – it won’t be overpriced compared to having it at home, and if people get hungry they can order their own meal.
Don’t put on a tab – everyone can buy their own drinks.
You don’t need to decorate if you don’t want but if you do just get some helium balloons from big w and scatter them around the room.

You definitely need to hire a venue of some sort, a hall or big room at a club maybe? Ask immediate family for donations towards hire costs instead of a gift?

How about a paintball party?