This is the thing that I always feel I need to get better at too! I always plan for them and they look good on paper, and then I run out of time.
But always formative assessment of LO (thumbs up/down/in the middle). Or talk to talking partner about something you have learnt in today's lesson. Sometimes I'll show a short film (BBC class clips, Espresso) or an animation/IWB activity (various sources including TES iboard). Or an application of what they've learnt (ie in Maths a word problem that allows them to use the skill you've done that day). Or use the time to set up tomorrow's lesson - get them a bit excited about what is coming next or establish current knowledge so you know where you're starting from.
I have a huge power point of plenary ideas that I probably downloaded from tes. I just put the word plenaries into the search box in the resource area and loads of stuff came up.