Keep it simple, and keep it inexpensive. He is not going to care about the theme. He will probably absolutely love playing Mylar baloons and watching them float up to the celing.
What I did for my son was make a Lego themed cake (made for cute pictures), had blue plates, napkins that said "1 year old!" on them, some fingerfoods, a few streamers on the ceiling, some Mylar baloons and invited close friends and family - Our gift to him was a nusery rhyme book that was a favorite of mine as a child.
Here is what happened - My son played with the balloons, stayed near the family members he knew best, and cried when he stuck his fingers into the sticky cake (he got tired earlier than normal due to all the extra actvity and was due for a nap). And he had a blast pulling wrapping paper off the gifts from friends. Overall it was about a 2 hour party and everyone had a great time.
His second birthday was lunch with family and a cake I made to look like a bulldozer - and later that week, cake leftovers with 5 of his toddler friends and their moms as a playdate. No decorations, no party favors, etc - but a lot of fun by all.
My point is, go ahead and have a theme and have some fun with the party, but don't plan a lot and don't spend a lot of money. Keep the elaborate birthday parties for when your children will be old enjoy to truly enjoy them. To start, go to a party supply store in your area, look at www.orientaltrading.com (lots of cute cheep stuff), and google "circus birthday party" I know there are lots of easy to make at home themed cakes you can make. Simple enough to make one for baby to dig into, and one for the guests.
State Certified Early Childhood Special Education Teacher
New Mom as of March 2009!