Good for you to help celebrate this new baby! Here are some thoughts, I hope they help or provide some inspiration.
Food: People who have asked to help- have them bring a dish. You could make it a pot luck shower and everyone bring something to share or just have cake a punch.
Invitations: computers generated invites are great and Factory Card Outlet has tons of stuff, very inexpensive, both invites or stationery.
Themes: I just threw a baby Library shower where everyone brought a special children's book to start this baby's library. It was a big hit. Another idea is a food shower, where guests bring frozen casseroles to sustain the new family when the baby arrives. Believe me, when I was a new mom, the thing I appreciated the most was when people would bring a meal. I would encourage the new mom to line up a circle of friends or people from church to help with meals for the first couple of weeks!
Location: 40 is a bit much for anyone's home (keep in mind the number you invite vs who shows up is about 50-60%) If you or the new mom belong to a church, they often will let you use the fellowship hall for little or nothing. Worth a phone call.
Centerpieces: Not necessary. (I didn't have them for my wedding!) It is something that the host or guest of honor notices and everyone else usually doesn't. Often expensive and not mandatory for a successful event. One idea is a stuffed animal at each table, or a votive candle or a single flower in a bud vase.
A gift idea: have someone who is handy with a camera take photos of the event, and then put them in an album and give as a gift to the new mom. I treasure the photos of events so much and I used to do them for brides as a gift as well. Computers and one hour photo shops make this a fun and easy thing to do.
Games: again not necessary, as most people want to visit and watch gift opening and relax. I have thrown numerous showers with and without games, and the ones with games are not as popular as the ones where we socialize and relax. That said, an ice breaker game is sometimes good, especially if you have groups of people that may not know each other. Funny name tags can be fun or even little slips of paper scattered at the tables with questions of how do you know the Mother to be or what is you favorite Momisms? (Sayings that were unique to your mom such as "don't do that to your face or it will stay that way!") You could have a book that is passed around and each guest can sign it with a pearl of wisdom, thought, wish or saying that relates to parenthood.
Biggest bit of advice: When people offer to help, say yes, even if you don't know what you want them to do. Give jobs that are suited to people's personalities or skills.
For instance: the talkative outgoing personality is a good person to serve food, greet guests or be in charge of the guest book. People who are good laborers may be great in the kitchen slicing and arranging. People who are artistic may be best to help set up tablecloths, decorations etc. If you need someone to do something and no one has volunteered, don't hesitate to say "Would you mind helping with the _______?" Most people are delighted to be a part of a celebration and like to have a task to make them feel they contributed to the event.
Rule of thumb: If something doesn't go as planned, don't stress out! It makes for a great story and funny memory later. People have the most vivid memories of wild things that happened that were not planned, like when my sister locked her keys in their running car in the valet lane at my wedding. That was 17 years ago and we still chuckle about it!
Also, if the hostess is having a good time, it means the guests are most likely having a good time. Try to enjoy yourself!
Hope this was a help!
J.