cake, soaked in something boozy to your taste...I like sherry or amaretto
then I fill any gaps with crushed amaretti biscuits
fruit....depending how sweet you like it then jam, my choice would be a bag of frozen rasps or cherries, very easy. And if you do want to sweeten them then heat them through with a little sugar.
more crushed amaretti or savoiiardi biscuits, more booze.
custard
NO JELLY EVER
then I put it in the fridge, it really wants to overnight in there to get a bit squelchy and juicy, I serve with double cream poured over, I think it's very easy for whipped cream on the top of a trifle to pick up that fridgey taint, so that's why I do not put the cream on before serving.
A blackforest type theme is amazing with kirsch and chocolate custard
More traditional would be rasps, and an almondy type custard.
But pick a theme and go with it, I always mean to do the Nigella Lemon Curd one, or there's a festive gingerbread one I saw somewhere but honestly I tend to stick with raspberries or cherries.