Stay in Bath before tour meets up

If all you plan to do is stay in a hotel, I might suggest you at least consider doing so in Bristol. Accommodations are likely to be less costly there, and it's a short 10-15 minute train ride from Bath. I'd recommend looking for a hotel close to Bristol Temple Meads station, where trains to Bath depart.

I've stayed at the Premier Inn Bath City Centre . Very nice, clean, quiet and a short walk to both the center of town and to the tour hotel (Brooks Guesthouse?)

I don’t think it’s worth the hassle of staying in Bristol for 1 night just to save a little bit of money.

Bath has a lot of decent well located hotels. The Apex or the Indigo always get well reviewed if you want something a level up form the Premier Inn.

Read Rick's chapter on a daytrip to Bath, it's a pretty well established list of stuff to see and do.

Also be aware that lots of people do that daytrip, so the city can be quite full during the day.

Frank II and Helen have given you excellent hotel choices.

Things to do in Bath...
Look at the things you will be doing with your tour group (The Roman Baths, a guided walk around Bath including the Royal Crescent) and choose something else.
I would suggest going inside Bath Abbey. Lovely architecture and sometimes a choir practice with lovely music.

There's also a bus ride around town that begins outside the door to the tourist information center. You can buy tickets in the TI. The bus goes up the hill to the east of Bath, around through the countryside, past The American Museum, and back to town.
You can get off any time.
There are 2 routes on this bus ride. We did them both. (Your ticket covers both.) This gives you a great overview of all of Bath and surrounding area.
If the weather is nice, sit upstairs on the open-top bus.

There are several good museums in Bath.
The Holborne Art Gallery is great. Walk over Pulteney Bridge to get there.

I agree with Helen that it's not worth a night in Bristol just to save money. But in my view it is worth it to spend some time in Bristol which I assume that you otherwise won't do. I personally think that Bristol is more interesting than Bath for the tourist overall.

Brooks Guesthouse in Bath.
Also, dine at the amazing Scallop Shell Restaurant.

Touring the Roman bath is a must, for sure. Also check out my favorite pub, the Boater. It's right on the weir.

We liked walking along the canals in Bath, and loved Bath Abbey.

Regarding Brian's suggestion....
sherrywildsc, the Roman Baths will be included in your Rick Steves tour, so there is no need to do that on your own before the tour begins.

For anyone reading this who will be visiting Bath on their own, I agree with Brian that the Roman Baths are a must.

In addition to what's already been suggested, you will not go in to #1 Royal Crescent so I would do that. You will see the Royal Crescent from the outside and this is a museum in a townhouse on the end. Very interesting view of Georgian architecture.

There is also the Herschel Museum of Astronomy (Herschel discovered Uranus and his sis was an astronomer as well).

If you want some outside time, I agree with walking the path along the River Avon to the Kennet and Avon Canal or if you want some leg work, walk up to Prior Park Landscape Garden to see the Palladian bridge feature. I also enjoy sitting in the Parade Gardens if it's a nice day!

Skip the Jane Austen Centre which is not where she lived and when I went years ago had nothing original of hers. There is a tea room there which I've not visited but if you want tea, I'd go to the Pump Room for that. There IS a Jane Austen Festival in Bath from Sept 13-22 this year. I timed my Best of England so I would be there that weekend and it was hilarious to see people walking around Bath wearing Regency costume. Very fun! If that is a time period that interests you there are also some special events for the festival that might be fun.

I'm glad you are going a day ahead! Good to allow for any travel delays and help you find your feet before the tour. This is an excellent tour and you'll see SO much!

...if you want tea, I'd go to the Pump Room...

We've been to the Pump Room for tea and it was delightful. Will do it again when in Bath.

I stayed at the Premier Inn before my Rick tour and found it very satisfactory. If you have never stayed in one before, it is similar to budget-moderate range chains in the U.S. In Bath, I even ate dinner that night and enjoyed my meal. I've stayed in several PIs in the UK and find although they are short on charm they give value for the money.