**Egypt **- I’ll grant some small quantities of gold for this location - but not as a cash substitute.  What you want to do is create gold works of art - jewelery, plaques, gold braid, gold wire for hair/clothing, gold cloth (yes you can do this), gold leaf for gilding things.  Bonus points for getting things created ahead of time that are exceptionally finely worked and detailed.  Double bonus points if it is an appropriate cultural reference to relevant deities or political figures of the time.  A well-managed “gift” to the priesthood or to the local governing agent can go a long way towards keeping you relatively safe and protected while you visit.
Healing is a bad idea here - it’s the province of the priesthood and linked to their deities.  If you go in and steal their thunder, you’re not going to last.
**America **- large hand-sewing needles (especially curved ones) and wire.  You’d be very well liked.  Make sure you create a cache to keep most of your stash, and don’t get followed when you return to it.  Be very clear that you only have on your person the little you’ve sold or bartered.
If you don’t mind fucking up history, bring lots of kitchen matches.  If you really want to screw things over but be very rich, find out from geologic surveys where those minerals are found and show them how to use them to create matches for themselves.  Not having to cart around banked coals for firestarting would be an immensely valuable thing to them.
Don’t go for the healing here either, for much the same reason as Egypt.
**Prague **- Here is where gold as “cash” will come in handy, however, you still don’t need much.  Before you leave, go hit up a large library and research merchants and guilds and make lots of small but targeted investments in the sucessful ones.
Other useful small things - medical needles are extremely hard to make and very useful - you could set yourself up as a medical supply dealer with just various bore needles.
On that note, you could try your hand at healing here, but be VERY careful about what you do, and who you deal with - you don’t want to fail with a notable patient (you’ll be accused of killing them on purpose) and you don’t want to be too successful overall (you’ll be accused of witchcraft).  If you can balance your number of poor people killed and not take important patients, you should be ok.