what is an alternative for baking powder?
Before chemical leavening, traditional cakes were usually made with one of three methods: (1) yeast, (2) foam created by beating eggs, (3) creaming butter and sugar together to create a fine foam.

It's generally much easier to choose a style of cake from one of these methods, rather than trying to adapt an existing recipe to work without baking powder/soda.


Yeast-based cakes aren't very popular in the U.S. these days, though certain traditional types still survive in Europe.  The problem with yeast-based cakes is that the additional rising time generally allows gluten to form, making your cake more "bread-like" in texture.  For advice on trying to make a more tender and lighter cake with yeast, you might have a look at this previous question.
As for egg foams, there are plenty of cake types which still use beaten eggs as the primary leavening agent.  As Joe mentioned, angel food and chiffon cakes are two obvious ones, but there are many variations on these types of cakes.  Many sponge cakes can be made without chemical leavening.  The general recipe format is to beat eggs (or sometimes just egg yolks) with sugar first to form a foam.  Sometimes the whites are beaten separately and folded in at the end, creating a very light texture, but these recipes often require care so they don't collapse before the structure is set.
Traditional pound cake is the most common example of the butter/sugar foam method, but there are lots of ways of making pound cake variants.  These cakes won't be as light as most egg foam cakes, but they can be richer, with a finer (but still lightly-textured) grain.


In addition, you also have the option of the many types of cakes which don't really "rise" much at all, including various "flourless" (or low-flour) cakes, "molten chocolate" type cakes, cheese cakes (if you consider them "cakes," which some people wouldn't), etc.