Take your partner for a long drive, with ice cream and good music. Enjoy each other's company and spend some quality time together. End the long drive by the beach, with picnic made especially for the both of you. 
Plan a treasure hunt that leads to you! Leave her a letter by her/him pillow early in the morning, with a clue to a place which is special to you. From that place, carry on the hunt, and let her/him drive around familiar places looking for you, encountering a friend with a gift on each destination. Meet your spouse at the final destination, with a nice lunch or dinner. Cook a special meal, with a candle night set up. You could customise it as you please, it is cuter when you order fast food like pizza and add a romantic touch to it. Like cut it into a heart shape, or write "I Love You" on it with cheese. 
Turn your living room into a campsite. Move your furniture around and build a tent right the centre, with a torch acting as your fireplace. It sounds childish but that's exactly the point, to feel like teenagers once again. Surprise your partner at their workplace. But be sure to get permission and the required off from their boss or manager. Surprise him or her with a bouquet of roses and chocolates, from there take them away for a date planned specially by you. 
Recreate your first date or first meeting. Go to that specific restaurant, mall or coffee shop and relive the moments that have led to this blooming relationship. 
Plan a surprise vacation for just the both of you. A secret rendezvous of sorts, plan a road trip to a nearby hill station, book a quaint bread and breakfast, shop for trinkets and cuddle. If you are a couple who enjoys adventure, plan a once in a lifetime experience for yourselves. Bungee jumping, deep sea diving, trekking, rock climbing, paragliding, canoeing, scuba diving and zorbing are just some of your choices to pick from, to make this day memorable. 
Leave signs behind throughout the day. Hershey's kisses on your partner's pillow, heart shaped soap and candles in the washroom. Even little love notes, reminding them of you, include poetry and dates of memorable days. So even if you aren't with your partner, they are constantly reminded of how much you love him/her. Plan a spa date at home, set up a bath with rose petals. It may sound clich but it always works. Decorate your room with scented candles and massage oils are always appreciated.