If you want to take over Monaco by force you'll have to deal with France. Essentially, because history, France tolerates a part of itself as a privately-owned municipality with a symbolic UN seat of its own. Similar things apply to Andorra though there it's both France and Spain, also they're more democratic but you'd still have a tough time with all that Catholicism there. Ibiza is part of Catalonia, ask the Catalans overall how easy it is to gain independence from Spain. Liechtenstein is also out, they actually gave their prince absolute power in a referendum. Hopeless case. By force, you'd soon discover that the mountains say "Grüezi". Vatican state, forget it.

There's a reason I mentioned San Marino.