Whether we use one or both of our bought tunnels which compress down to 'nothing', or whether we cover our clothes drying horse, a line up of a few chairs, or our coffee table with a sheet... tunnels have been an easy, fun way to entertain our toddler (as well as expel some of his energy) when we have been stuck indoors during cold, wet, wintery days. In summer, we'll definitely set them up outside every now and then too.
It's important to note here, that the whole 'tunnel experience' is not every child's idea of fun. It can be overwhelming and scary. Master B. was initially hesitant about entering the tunnels I'd set up, and is still not convinced about the blackened/dark one, but without any pressure, slowly his confidence has been improving, and his curiosity often gets the better of him where he wants to find the toys strategically placed along the length of it, and also play with the toys on the other side.
Tunnels can also provide some degree of childproofing - blocking access to doorways they cross. My favourite bit is that they can easily be packed up and stored away after a day's play!