One of my favourites is playing hard/impossible with huge galaxies, but using only the starting systems. You are allowed to colonise existing planets and construct planets, but you cannot colonise a new system.
I do this for 2 fold:
1) I'm so tired of manageing colonies over and over, i always select the same stuff genearlly in the same order. The game goes into a crawl much later becasue of this. Maybe I should play with letting the AI build automatically, but it tends to build horribly.
2) It permits a faster game, which I prefer.