If you make a cake ahead of time, what would be the best way to warm it up to serve without drying it out? Microwave or 350F oven for a few minutes?

I agree with @takeanonion. Wrap the cake in aluminum foil and gently reheat in the oven. I would set the temp at ~300-350F and heat for about 15 minutes. The foil will keep the cake from drying out. I've made the Swedish Visiting Cake from Dorie and it's dang good at room temperature too.

I'm a fan of pre-heated high oven (400+) for 10 minutes.
Cake integrity seems to suffer when heated in the microwave oven.

just a simple chocolate cake or dorie greenspan's swedish visiting cake? so a simple cake that can be made ahead of time and warmed up/ served with ice cream

What kind of cake are you planning on serving?