How should I surprise my partner sexually?

Sex is one area where surprises are generally a very bad idea. Many relationships have ended because one partner thought the other would enjoy x, only to find out that they were disgusted/insulted/traumatized/etc instead. It is difficult and may be impossible to recover from this type of oops.

The best idea is to set aside some relaxed time to discuss interests, curiosities and hard limits with your partner. Probably start with the hard limits so you don't stomp all over them with your interests. Then decide to try out one thing you both are interested in. Keep communication open at all times. Go slow. Keep in mind that some (many) people have been sexually assaulted or abused in some way and that will make them incredibly sensitive to anything that recalls those feelings.

You want this relationship to be as enjoyable as possible for as long as possible? Yes? Don't take chances, be smart, be kind, communicate openly and good things will happen. I promise.

I cannot stress this enough.

Under no circumstance should you attempt to surprise your SO with a previously undisclosed sex act. You should also avoid any unsolicited digits entering orifices.

A finger in the butt is always a nice surprise.

talk to them about things they like and things you like and see what overlaps (if you haven't done this I will presume you're heterosexual)

then, prep ahead of time to do one of those things at a time when you think it's likely your partner will be receptive to it (not after a long day of work)

hint a little about it ahead of time and use that to judge whether they are in fact probably up for it

bring them to the bedroom or other fuck spot and explicitly ask if they're interested. if they are not, make it clear that it's okay (and mentally prepare yourself for it to be okay)

Not sure what to say other than "try something you haven't before or haven't in a long time". Maybe it works maybe it doesn't, but it's good to change things up now and then.

You could also ask them if they have anything they'd like to try, maybe suggest some ideas too.

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.

You have given zero information that we need to offer relevant advice. You haven't told us your gender or the gender of your partner, what your ages are, what your respective experience levels are, what you normally like to do, etc...

Any specific advice anyone tries to give you would be nothing more than throwing spaghetti at the wall and seeing what sticks.

If you want good answers, you need to provide more information.

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

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