A hotel room and some activities not involving sex. Maybe it’s a show or going to a garden or a museum. Make it a total romantic evening and set the stage. Then as others have suggested maybe you give a gift of a sexual nature like some sexy clothes or a toy. That way it won’t be a surprise act but something they can choose to use if desired. Make sure it’s all no pressure and good natured fun.

If it’s a toy, before engaging in intercourse say “Look what I’ve got!”

If it’s an act, first ask if you may surprise them in intercourse, then once you have that consent be extremely ready to stop everything if they don’t like it. Remember that consent can be revoked at any time and it’s important for any sexual partner to respect that

As mentioned before don’t try anything too crazy without explicit discussion and consent. I really recommend Hot and Unbothered if you want to work on your sexual communication.

But if you want to get started before the book I recommend a Yes, No, Maybe list. Is is pretty common recommendation from sex positive therapists and kinksters. It’s basically a long list of every sexual thing you can think of. Without judgement you mark which ones you are Yes, No and Maybe about. Then you have a common list of new things in the Yes category you can try and some aspirational ones in Maybes. There are some templates online if you want to use them as a source

A finger in the butt is always a nice surprise.