Memory Lane
Idea suggested by Alejandro Chávez.
This idea is perfect for those times when your partner will be away from home for a while. These moments of solitude not only allow you to enjoy some 'me time' but are also ideal for planning a surprise.
First, you need to know exactly when you'll have the house to yourself. Your plan should be achievable within this timeframe. Be realistic with your planning so you don't get caught in the act under any circumstances.
Next, think about the path your surprised partner will take when they arrive home. It's always lovely to mark the path with petals of their favorite flower, chocolates, candles, or even lipstick marks on the floor (kiss the ground they walk on). Another great idea is to add cards along the path with stories and anecdotes you've shared together: those unforgettable vacations, that pet you bought, etc.
The end of the path should be designed based on how your partner will arrive. If you expect them to be tired, perhaps you could end with a prepared salt bath; if they're hungry, a table with candles and a nice dinner will be perfect. If you sense passion might be in the air, prepare the bed accordingly... In any case, don't forget a final touch like a bouquet of flowers or a letter declaring your eternal love. Let your heart guide you to the best option for each situation.