I just recently helped throw a shower. The co-host had planned a budget of around $250 for the whole thing and I think we came in considerably under that (out of town grandmother-to-be was kind enough to pay for it, so co-host was the one keeping track of costs to pass along.) Not a co-ed shower, but was one of the biggest I've attended as far as guest count. Even with that number of guests, we over-stocked on food and probably some other things could've been done differently to get it even cheaper than that if we had needed to.
-Theme: This one was ocean themed, since that's what the parents-to-be are going with for most of their baby stuff. I like the book themed ones too where you're helping get baby's own little library started. Theme is pretty optional though I think, it can totally just be a "Baby shower" theme. :)
-decor: Aside from sea-colored table cloths, we had a fishing net on one wall with baby items "caught" in it. (This doubled as a game: partway through the shower when people were distracted we took all the baby stuff down and then we asked them to try and list as many things from the net as they could remember.) Also made some paper lantern jellyfish (pinterest find) that the parents liked enough they now have some of them hanging in their room near the changing table. Some seashells strewn around on any table space that wasn't covered by food... I think that was all of it. For the baby stuff we hung up, we just used items the parents-to-be had already received since many people sent gifts early if they weren't going to attend the shower, so that didn't affect the budget. (Although we had the advantage of throwing it at the couple's apartment complex community room, so I was able to raid their stuff more easily than you might be)
-other games: we had more planned but only ended up playing the memory one above and then the one where you give people crepe paper or ribbon and have them cut off how much they think is the right measurement of mom-to-be's belly. (Best to double check with mom-to-be if she'll be offended by this though! ours thought it was great). Good to have a few games prepared, but also read the party to see whether they're really needed. We had to struggle to get people's attention for the two we did play, any more would've been intrusive since people were enjoying chatting. We had already set up a raffle for the spare prize we had on hand, so we just drew a couple extra names to give away the others from the games we skipped. Some other okay game options from the co-ed showers I've been to: baby-themed pictionary, team scavenger hunt amongst our belongings for items related to stories about the couple, best 60-second baby sculpture out of bazooka bubblegum (no chewing allowed) or small piece of playdough, speed-folding baby clothes contest for the guys etc etc. But for everyone's sake, don't do the one where you melt candy bars in diapers and make people guess which kind... that one is pretty universally hated. :) (oh and just saw the post from Ladystache, +1 for bingo during gift opening)
-food: local grandma-to-be is a beast in the kitchen and made lots of tasty stuff, mostly small sandwiches (this may not be the exact recipe she used, but "funeral sandwiches" were good:
http://www.cleverhousewife.com/2013/09/funeral-sandwiches-nothing-mourn/). And then we had the usual party grub standbys of fruit salad, chips/guac, hummus and veggies etc, much of it from Costco.
Per mom-to-be's request, I made these (although I did mini's) for dessert:
http://cltampa.com/dailyloaf/archives/2009/04/24/sweet-indulgence-vanilla-bean-cupcakes-with-blackberry-buttercream. They are beautiful and super tasty, but be warned it makes over 100 if you do mini ones. I only make them for special occasions where I have lots of help eating them! I also made some easy sand dollar sugar cookies (pinterest strikes again) and we had madeleines (since they're shell shaped) and brownie bites from costco.
In my limited experience, it seems like the co-ed showers have usually been a lot more laid back about theme/decor/games so as not to freak out the guys who aren't used to attending stuff like that. So I think as long as you host a nice party where everyone has fun and gets to celebrate the parents-to-be, you should be in good shape!
