Java EE 6 comes with EJB 3.1 and CDI. You could work exclusively with EJB 3.1 or CDI, but you will not end up with the simplest possible solution. CDI comes with typesafe and scalable (=start easy, but sky is the limit) DI, events, decorators and "glue" logic.
CDI, however, is missing some EJB 3.1 features:
- Asynchronous, transactional execution with Future support
- Pooling: important for throttling and prevention of denial of service attacks
- Monitoring - all EJBs are exposed via JMX and can be so monitored with JConsole
- Declarative, "Convention Over Configuration" transactions. Without EJB 3 you will end up in a considerable bloat
EJB 3.1 and CDI are both part of Java EE 6. The "KISSiest" thing you can do is to deploy a EJB 3.1 boundary and use e.g. CDI behinds behind.
You could also implement all aspects (transactions, jmx, throttling) by your own - but this would introduce additional complexity to your project (and: don't re-invent the wheel).