Here is the full list of all 36 Tony Hawk's Pro Skater 1 + 2 achievements.
Complete all "Hard" Get-Theres.
Find every alien plushie.
Earn 5,000,000 points in Trick Attack across all playtime.
Earn 2,000,000 points in Combo Mambo across all playtime.
Paint 100 surfaces in Graffiti across all playtime.
Find all Stat points for any Pro Skater.
Unlock Officer Dick.
Earn the "Veteran" Challenge Collection.
Earn the "Legend" Challenge Collection.
Complete all Gap collections.
Get hit by the Bull in Bullring.
Complete every goal in THPS1 Classic and gain all gold medals.
Complete every goal in THPS2 Classic and gain all gold medals.
Unlock Bullring in THPS2 Classic.
Get hit by a vehicle on every map that features them.
Achieve a judge score of 99.9 on a competition run.
Earn any 4 Medals on competition parks in Tours.
Unlock School II in THPS2 Classic.
Spend $10,000 in the Skate Shop.
Earn a score of 10,000,000 in any park in a 2 minute session or single combo. Game Mods must be disabled.
Land a 1,000,000+ point combo.
Achieve a High Score of 1,000,000+ on any classic Park.
Unlock Roswell in THPS1 Classic.
Give 50 Letters in Horse.
Complete a match in multiplayer.
Earn the "Rookie " Challenge Collection.
Earn a Medal on any competition park in Tours.
Break 20 Panes of Glass.
Create a skater using the CAS system.
Reach Career level 10.
Bail a 100,000+ point combo.
Land a 50,000+ point combo.
Unlock School in THPS1 Classic.
Create a new park and upload it.
Place 500 objects in CAP.
Reach Career level 100.