The most important requirement is (both) people have to want to spice things up!
Unfortunately this is rare because in most relationships one person is usually "content" with the way things are. In some instances the content mate will make an effort or go along with the one who desires to bring back the spice for awhile. However overtime they will revert back to what they consider to be "normal". Very often these people see their mate's desire for spice or more romance to be "unrealistic". The biggest challenge for couples is having (both people) feel the need to mix it up or make changes. Going along to get along won't last long.
Having said that if both people are on board the first thing to do is rediscover what you both enjoyed doing when you first got together. Maybe there were places you have not visited in years. Weekend getaways are fun as well as tropical island vacations, even spending a Saturday downtown in the largest city near you doing what tourist and visitors do complete with spending the night in a nice hotel.
Some less extravagant things a couple can do is go back to flirting with one another, use sexual innuendo with one another, leave love notes on the bathroom mirror or in each other's cars from time to time, take showers or baths together, cook a meal together, host a small dinner party or BBQ with 2-4 guest, take walks together in a local park or a stroll along the beach, give each other massages by candlelight and music, take a fun class together, dine at a restaurant that has live music, check out local activities and festivals taking place, during a storm make some popcorn and watch some old movies together or play a romantic board game, visit a large adult store...etc One is only limited by their imagination!