to fill the available space

Were it not the case that he could go from the table to the windowsill I'd be happy to allow the table to be a stand-upon-able thing, but the windowsill isn't particularly sturdily-braced and is quite narrow. The table is much more sturdy than the ActivityCube® which is perhaps why he constantly wants to climb onto it, even a couple of seconds after lowering his knee after recognising that we're asking him to get down from it.