You should check the Ugandan embassy in Germany for the right information as a German citizen (I assume?).
A single entry visa is valid for three months and is only good for one entry.
A multiple entry visa is good for six months (or more, depending on what you pay for) and will allow you to cross the border and return. If you are staying for any length of time and think you will want to do this, you should get a multiple entry visa. The Ugandan High Commission in the UK (http://www.ugandahighcommission.co.uk/ConsularInformation/VisaInfo.aspx) says you must apply for longer multiple entry visas from Ministry of Internal Affairs in Kampala.
Although you can get both on arrival I think it costs less to get the multiple entry visa in advance.
I wasn't asked to show a return ticket when I visited Uganda a couple of weeks ago, but I was asked to declare how long I was staying in the country. If you don't know how long you are staying you might have different requirements.
