I think that a great set for US gold collectors is US $1 Gold coins that were minted in San Francisco, see https://www.ngccoin.com/registry/competitive-sets/119601/ 

US $1 gold coins were minted at the main Philadelphia, Pennsylvania mint (no mint mark) and the branch mints, i.e., the Charlotte, North Carolina mint (C), the Dahlonega, Georgia mint (D), the New Orleans, Louisiana mint (O), and the San Francisco, California mint (S). 

Below I list the number of dates for each mint for each of the different types of $1 gold coins. I also list the individual dates for the San Francisco mint since this is the set that I am focusing on in this post.

•    Liberty Head (1849-1854)
     o    P – 6 dates
     o    S – 1 date, 1854S
     o    C – 5 dates
     o    D – 6 dates
     o    O – 5 dates
•    Indian Princess Head, Small Head (1854-1856)
     o    P – 2 dates
     o    S – 1 date, 1856S
     o    C – 1 date
     o    D – 1 date
     o    O – 1 date
•    Indian Princess Head, Large Head (1856-1889)
     o    P – 24 dates
     o    S – 5 dates, 1857S, 1858S, 1859S, 1860S, 1870S
     o    C – 2 dates
     o    D – 6 dates
     o    O – 0 dates

There are several attributes that make the S-mint $1 coins a great set to collect:

•    There are only seven S-mint coins in the set.
•    There are three different coin types represented in the set.
•    Back when I was collecting these coins between 2008 and 2012, the S-mint coins were much cheaper than the other branch-mint coins.  The Philadelphia coins were the least expensive.
•    All the S-mint coins are scarce, but they can still be found in uncirculated grades.
•    It’s possible to complete this S-mint set because of the attributes listed above.
•    There is a lot of interesting history associated with the S-mint coins because of the California gold rush (1848–1855) and the opening of the San Francisco mint in 1854:
     o    California gold rush https://en.wikipedia.org/wiki/California_gold_rush 
     o    San Francisco mint https://en.wikipedia.org/wiki/San_Francisco_Mint#:~:text=Opened in 1854 to serve,current one%2C completed that year.

I never completed the S-mint $1 gold set (I own 4 of the 7 dates in this set) because I changed my collecting goals in 2012 (see my previous posts). I show the obverses of five US $1 gold coins in the header of this post. Below the header I show the corresponding reverses of these coins.

Since I don’t own a 1854S, I show an 1851D (Dahlonega Mint) in the far-left slot to show an example of the Liberty Head type.  The second coin is a 1856S, which is an example of the Small Head, Indian Princess type. The remaining three coins are examples of the Large Head, Indian Princess type.

•    1851-D, Type 1, $1, MS-63 (NGC), (152 total, 5 in, 7 better)
•    1856-S, Type 2, $1, MS-61 (NGC), (250 total, 15 in, 13 better)
•    1857-S, Type 3, $1, MS-62 (NGC), (121 total, 4 in, 2 better)
•    1858-S, Type 3, $1, MS-61 (PCGS), NGC Census (113 total, 8 in, 4 better)
•    1859-S, Type 3, $1, MS-60 (NGC), 163 total, 6 in, 11 better)