A quick calculation tells me that out of your 10 days you’ll be spending at least 3 days / 2 nights on trains. (I estimated a non-stop trip Madrid – Barcelona – Venice – Rome – Paris – Amsterdam without even visiting the cities. Leaving Madrid on Monday would get you to Amsterdam on Wednesday evening the soonest.) This leaves you (in theory) 7 days for actual visits.
Maybe it’s time for some re-planning…? Maybe consider taking some flights (e.g. Barcelona – Venice or Rome – Amsterdam).

And I don’t know what ticket has cost you only 100$ for two, but I can assure you that you will not get a romantic 2-persons cabin on a sleeper train for that amount…