Put a good layer of grit in the bottom and a mix of john innes potting compost (the one designed for potting on NO 3 I think and sand 4 compost to 1 sand plant with houseleeks in different colours add in some miniature bulbs for the spring things like chionadoxa, and just a few mini daffs add a couple of attractive chunks of stone slate is nice and a mini conifer (you will need to replace it every couple of years as it will get too big) and mulch the surtface with gravel
A min Zen garden is an option too Just fine gravel and nicely positioned rocks with maybe the odd clump of something green like a selaginella and maybe small conifer pruned to look like a bonsai tree but that will need maintenance
It looks really nice if you leave a 'stream bed' of gravel with a few larger pebbles running through the planting scheme Did this in a large shallow trough when I had a roof garden and surrounded the planter on three sides with ferns and hostas to frame it