The Open House
Just remember that an open house style format means that people will come and go as they are able.
You get way more personal time with your guests instead of everyone vying for your time at once. I see your concerns though. With an open house, I see it as open, meaning anyone can come. But you typically see people stay for a shorter period of time with an open house, and I would be surprised if you had everyone there at once. What does your mom think of it? I think she should be the person you listen to the most because she's throwing it.
I think it's a fantastic idea though, and if I was doing a shower this time around I would definitely steal it. But my guest list would be closer to 30 and I personally have enough space to do it.
Revolutionary War Baby Shower
Get some hoop rolling going for the kids and adults! This was a past-time of kids way back in the day. Get some fun accessories and set up an area for pictures:
- three cornered hats
- fake muskets
- red coats
Instead of a guestbook, have a parchment like paper and have everyone sign at the bottom (like signing the declaration), you could print the declaration at the top, or make your own.
Many baby shower games are silly but you could have a trivia portion (with prizes) or at least some fun facts printed out. Or just leave trivia cards on tables for people to play with/ look at while eating.
I've seen team building exercises where people construct a flag with one person blindfolded and two people directing the construction. Use the revolutionary flag!
Have everyone sign a star or a stripe with well wishes for the baby, and construct a flag (would be cute if they were fabric and you had then sewed together).
Make some beanbags that look like tea bags or cannon balls and have a bean bag toss (corn hole) type game.
Have food with a theme - tea, macaroni salad (Jefferson brought back macaroni from France to introduce to the colonies), cherry pie (for Washington), etc
These are great for fun themed shower that might not be mother's first.
The way I've seen it work is that the second baby shower is an excuse for people to buy you cutesy gendered stuff, but also have more fun than the first time around.
If your second baby is a girl, then you get a bunch of floofy pink clothes and toys. If it's a boy, a bunch of blue clothes and boy-themed toys.
People don't generally register for or buy big ticket items like car seats because, yes, you can reuse them despite the gender difference between the babies.