I like the default theme, Twenty Eleven a lot because it is very flexible and easy on the eyes. The only thing that this theme lack of is the ability to set the sidebar to appear on the blog posts.
I have discovered a plugin recently that will allow you to display the sidebar on the posts as well as the pages. The plugin name is: Twenty Eleven Theme Extensions. All you need to do is to install the plugin and activate it. Next, go to the theme extensions and set the sidebar to be visible for posts and pages. Its that easy!
Besides the sidebar options, this plugin also offers other features, check it out!