Often pretty simple stimuli can add quite a bit of engagement. Free Game Show Soundboard (found at this link or in App Store under iPhone apps) is just a screen of buttons that, when tapped, play sounds such as a bell ding, applause, audience laughter and cheers, etc. The app also has countdown timers and scorekeepers for two players.
I try to avoid too much competition in my sessions, however, this app can be used to add engagement and reinforcement when students are involved in tasks of earning points or naming items in teams, or just as a fun way to let students know what they are doing well...or with a twinge of humor, maybe not quite as well. In conversational activities, the "applause" serves to alert students when they have made a good comment that supports others, and I especially like to use the "crickets" to humorously point out a silence that students need to try and fill with a "thought!"
If you like the idea of using sounds in your sessions, there are a wealth of other soundboard apps that can be used in similar ways, to build listening skills, in units about the five senses, or to relate to narratives- this developer also has soundboards related to Halloween or farm sounds- relevant to this time of year!