2 daytrip to Switzerland where to go?

1) Basel (because it is close).
2) Luzern. Take a trip on the lake.
3) The Rhine Falls near Schaffhausen
4) Bern

For routes and times see www.viamichelin.com
Neuchâtel and Schaffhausen are worth a few hours only, not a day trip. Zürich is not one of the highlights of Switzerland, and would involve changing to Public Transport to get into the centre.
Lots of other places if you want snow and glaciers (Titlis, Grindelwald, a circular route over passes), but all would break your 2½ hour limit.

Don't forget you will need a Swiss Autobahn Vignette (if there is not one on the car already). Cost 40 Franks

Thanks Chris! Would a rental car come with the pass you're speaking of and if not where would I need to buy one?

Schaffhausen and the Rhine Falls are nice but the really neat town in that area is Stein-am-Rhein. It is a walled town on the Rhine that has been there forever. If you check it out on Google Maps, it meets your 2.5 hour driving range and you can pass through Schaffhausen on the way. Given my druthers I'd pass Schaffhausen but that's my preference. It is relatively small and easily doable, just park outside the wall and wander the town. Not only does it have a ton of great old half-timbered buildings but many of them have frescoes on them that are just plain fantastic.

Also the Restaurant Klosterstubli near the bridge is a place we found on our second visit and started our love of Rosti. It is a low key non touristy place that we will get back to sometime.

Lausanne is a beautiful town. On lake geneva with a chance for the kids to be on a restored historic paddle wheel cruiser. The surrounding vineyards are picturesque.chateau chrillon a nice small castle to visit nrby.

Kate, you should chat with your rental agency about vignettes. They might give you one, or sell you one, or tell you how/where to get one cheaper.

Thank you all for your suggestions! Looking forward to our trip!

Kate, a Swiss Autobahn Vignette is a toll for using Swiss Autobahns (=motorways=Autoroutes=Autostrada). It comes in the form of a sticker which you stick on your windscreen, costs 40 Franks for a calendar year (shorter periods ARE NOT available), and looks like this.
If you hire a car in Switzerland it will already have a sticker on it. If you hire a car in France it probably won't, unless someone has taken the car to Switzerland earlier in the year.
When you cross the Swiss border on an Autobahn, there are two lanes, one for cars which already have a Vignette, and one for those that do not (see sign here). If you don't already have a Vignette, you go in the correct lane and they sell you one.
The Vignette is only valid once it is stuck to your windscreen and cannot be removed.