There is definitely not a case for a police. While "good cops" will be distracted from real crimes by this. A "bad cops" will see here solution for a dozen of unsolved crimes. The both are bad for you.
Although, if my near and dear done something comparable, i'll be more mad that the devil itself. It is not a subject for jokes at all.
But if a girl's mind found thoughts of kidnapping in itself, maybe she would better try it on herself?
OK. OK... The following is pure fantasy. I don't know you, your gf and yours relationship. So think of it just as a thought experiment.
First. Make her wait. It should not be today or tomorrow. The anticipation and uncertainty makes every punishment 100x better.
Second. You mentioned that you don't have much implements/devices. Great! Make this on her budget.
Third. On the Time she should have some deals that she care about. Not critical, but unpleasant to miss.
4. You should be emotionally cold during this session. Don't talk to her at all or say only monosyllabic phrases.
5. Vision deprivation. Not a blindfold or a sleep mask. The blacked-out swimming goggles is the way to go. It is hard to undone and uncomfortable for a long time. Yet relatively safe.
6. Palms restraints. Ability to move fingers is a way to escape. Close it at the beginning. Wrap hands by something like vetwrap (self-adhesive tape).
7. Body restraints. Rope is great. But not for more than hour or two. The body should be able to move, but in a restricted way. The best for it is steel cuffs and chains, additionally attached to a wall. The additional effect - you will listen when she moves. The cage is also great but expensive.
8. Clothes. There are two options. No clothes at all. Or something simple and soft.
9. Drinking. It is very inhumane to forbid person to drink. So she should have this ability in most humiliating way. The hanged bottle with a calf pacifier.
10. Toilet. There is no such thing as right to use toilet in any time. Catheter all the time and enema twice a day.
11. Gag. For a long time it is a bad idea. But in case of complaints and moans there should be one.
12. Temperature. It should be cold. Not freezing cold, but uncomfortable cold. It is about 15C (60F).
13. Feeding. The most severe way is feeding troughs a tube. But it needs some experience to perform. So the dental gag is also a way to go. It is very uncomfortable an inconvenient to swallow with it. So use small spoons of soft food without any powder.
14. Cold shower is a best way to deal with complaints.
15. Distraction. This is more of a torture than kidnapping. The person should not have ability to concentrate or relax. The best way is to make her solve simple tasks in her mind. Like arithmetic. No answer for 20 seconds or wrong answer - electric shock via butt plug and/or loud unpleasant sound. Todays technologies allows to automate this via simple scripts.
There how kidnapped person should be cared to not to escape. Not that cinema bull**it.
Heh... Even reading all of this will be enough punishment for some persons...