You have 30plus children in a snake, ready to walk to assembly and time on your hands.
Poetry can be a great filler. I always had a set of poetry books on the shelf, to share regularly, and could then choose as few or as many as needed to fill in the available time. It’s especially useful if the children know many of the poems my heart, so can join in with the telling, as a rehearsal activity. Children like silly poetry.
They also like silly songs, so a collection of these would be developed through the year, so that, with a few minutes and a child chosen, the line could become an impromptu choir, enjoying the feeling of performing together.
The numbers 1-100 written on separate cards allowed two or three to be selected at random, with the chosen numbers discussed by the children. This allowed discussion of larger/smaller, or greater/lesser, place values, ordering a set of numbers according to attributes. It could also become random mental maths challenges with addition, subtraction potential.
Dice come in different number combinations. These can be thrown to create random maths problems.
I’ve even used the Dienes base 10 material, challenging children to find specific values; 36= 3 tens and 6 ones.
This could be extended to broadening vocabulary needs, where a word selected at random is created into a sentence, and improved around the class. This could be subject specific vocabulary, with the need to incorporate other knowledge into the sentence.
I also had three piles of cards, divided into people, places and things. Children would take one card from each pile, then create a story to link the three elements within the time available. This style of activity was occasionally developed as a writing stimulus, for longer consideration.
Images. A classroom collection of images, as paper copies, or through the IWB becomes a talking point, starting from description of what is seen, through speculation about what happened before and after the image was taken, or what was happening outside the frame of the image.
Five minute slots can be very important for rehearsal activities. It is far better to have ideas as a back-up plan, as five minutes can sometimes extend and a line of children standing around can get very restless, developing into a behaviour management issue.
My advice is always to be prepared. Have something up your sleeve, especially if you can do magic tricks!
You might like to look at one minute data ideas or five senses starter activities