First I would decide on a theme. I've hosted four baby showers, all of varying sizes and themes and I like to nail down a theme so that I can plan the games, food, and favors around it. I've done a literary theme, which was my favorite so I did it twice, a coffee theme and scrapbook theme. The coffee one was really fun too! The mom to be LOVED coffee, so of course I had decaf available but it was cute. Not to be mean, but I really don't like those decorate the onsie parties. In theory it's a cute idea, but honestly, some of them (I've been to a few of the parties myself) are really cute but most of them are just weird and not something I'd want to dress my kid in! I know that probably sounds bitchy, but the supplies for that would be expensive and I just don't know if I'd want that if I was the mom being honored. Bibs, as someone else mentioned, however, would probably be better. I haven't seen that before and I like it better.
Anyway, as far as games, the basics of like tummy size and the chocolate diaper game, although slightly in bad taste, are really fun! I also made up my own trivia game and looked up a bunch of questions and answers on the internet like how many diapers do you go through in a year, most popular brand of wipes, stuff like that..and it went over pretty well. Honestly, I google searched baby shower games and sort then did a pick and choose of the stuff I liked the best. It's kind of overwhelming, but you can eliminate the ones you wouldn't do pretty quickly.
Not a game, but I also made my own diaper cakes which were super easy and looked really cute as the centerpiece. Plus it was part of my gift to the mom. If you google them you can see what they look like and how to make them.
If you have additional questions please feel free to PM me. I love a good theme party and try to be full of ideas! ;) Have fun!
Ooops...I wanted to add onto Amanda's suggestion about guessing the candy in the baby bottles...I saw that at one of the showers I was at too and it was really cute. They actually had about 10 bottles set up with random stuff in them, mostly stuff for baby or mom. Some bottles had Q-tips, or cotton balls, bandaids, Cheerios, etc. It was really cute and it was a game that you didn't have to lead. YOu could just leave the game out and encourage people to guess. They had a basket by each bottle and you had to put a slip of paper with your name and a guess on it. I think the winner from each bottle won a prize but you could also do the person who won the most could get a prize to cut down on your prizes if needed!