dinner party at a posh restaurant? tell all your friends to not get you presents and just pay for their meal?

instead of going to the spa, go to the mall to some stor that sells spa stuff (like bath and body works) and do your spa at home. well, since youre already at the mall with your friends buying spa stuff, woulldnt you wanna do some other fun shopping with them too? Given. i havnt had a decent party in years and that would be my dream party

What about going to see your favourite band on concert?

I went to a hotel in London for my 16th Birthday with my best friend and they had a Spa there. I'm sure you dont have to be 18 for them all. We didnt didnt get asked when we booked or when we arrived and we looked our age!

just lie. tell them that you are 18. and make fake ID cards. I have seen that in movies a lot.