A couple ideas would be to make quick hand-print or foot-print cards for the parents to take home. You can use face paints (my favorites are the kind that look like a tongue depressor dipped in paint) or finger paints and then put their child's print on a card or small poster board with the date and their name. Make sure to have some handi-wipes handy to quicky de-paint the hands/feet. Some wee ones (ittsy bitsy babies for example) respond better to having their foot print made than a hand print. White prints on black paper also look cool. It can be neat to get Mom/Dad's next to their kiddos too.
Another small idea would be to make them CDs of some of your favorite kid's music (or music you listen to). You can call the albums "So-in-so's Birthday Album". I always like getting to know about new artists since I'm kinda dorky and I don't buy music very often. (I still remember how much I liked getting mixed tapes from boyfriends.)
As for games, I think it's fun to have a bubble machine at this age, and if it's hot they could wear thier swim suits and splash around in the pool.
Also, you can spread out a big roll of paper or poster board and let them "paint" with using finger paints. Encourage them to use their hands, feet,elbows etc and paint the whole thing - and each other! Then hose the kids down. Ofourse the latter requires the swim suit attire also.
Have fun!
~J.