Hi. I am traveling to Tel Aviv and have the option of staying to days on my way to TA in Bucarest or Paris or 4 days on my way back in either cities. Do any of you have any recommendatios on time to spend, expense to consider? I am travelling alone
I have been to Paris before, but do not know if there is a lot or a little to see in Bucarest or the surrounding areas. I am very fast when travelling so can probably see the double of things most people see in the same time frame.

Could you male it Vienna or Budapest instead of Bucarest?

There is lots to see in Bucharest.

I was there for five days last year.

I stayed at the Hilton right across from Revolutionary Square and had a room facing the Square.

I also did a full day Castle Tour outside of Bucharest which included Bran Castle ( Dracula )

Stay one-two days in Bucharest itself then catch a train to Brasov in southern Transylvania (about three hours away, tickets from €11) and spend the rest of the time there.
In that area there's plenty to see/do for a few days (medieval cities, castles, old villages, the Carpathian mountains etc.).

As mentioned above, do Bucharest (night 1) then Sinaia on the way to Brasov (Bran) (night 2 or 3 if you spend day 2 in SInaia and arrive in Brasov in the evening). By train you could extend this to Sibiu and sleep there night 3, or 4, depending on what you decide.
3-4 nights should give you a taste of Romania.

thank you all for your helpfull advice. should I consider it to be expensive?

It's not very expensive, unless you plan to use 3 to 5 star hotels.